THE TRACK CIRCUIT
"Perhaps no single invention in the history of the development of
railway transportation has contributed more towards safety and
despatch in that field than the track circuit. By this invention,
simple in itself, the foundation was obtained for the development of
practically every one of the intricate systems of railway block
signaling in use today wherein the train is, under all conditions,
_continuously active_ in maintaining its own protection.
"In other words, the track circuit is today the only medium recognized
as fundamentally safe by experts in railway signaling whereby _a
train or any part thereof may retain continuous and direct control of
a block signal while occupying any portion of the track guarded by the
signal_."
"To Mr. William Robinson the Patent Office records concede the honor
of having devised the first practical track or 'Rail circuit'. This
comprised what is termed the _closed_ track circuit. * * * Closed
track circuits are very reliable, wholly safe in principle, and simple
of application and maintenance."
The above paragraphs, quoted from the third annual report of the Block
Signal and Train Control Board to the Interstate Commerce Commission
under date of November 22, 1910, ably express in a few words what the
invention of the track circuit has meant to the railroads of this and
other countries. In order, however, that those who are not familiar
with the principles of the track circuit may have some general
knowledge of them, a simple, non-technical description is given, as
prepared some years ago by Mr. J. P. Coleman, of the Union Switch &
Signal Company.
Historical information on the development and use of direct current
and alternating current track circuits for roads using electricity for
propulsion purposes and those using steam will be found in a report on
this subject made by Committee X to the Railway Signal Association in
1910.
The Rail Circuit Principle
By J. P. Coleman.
Assuming that it is clearly understood that the current is generated
at the battery; that it flows from thence through the conductors (of
which the coils of the magnet form part) and back again to the
battery, and that the magnet is simply a device interposed in the
circuit for the purpose of transforming electrical energy into
mechanical (magnetic) energy, and that the latter can exist in an
electro-magnet only with the presence of the former, we are now
prepared to make clear the principle of an electric track section.
To assist in this, let us state an invariable law governing the flow
of currents: _If two or more paths be presented an electric current,
it immediately becomes divided, and flows in each in quantities
directly in proportion to the conductivity of each._
The unit of electrical resistance, whereby the comparative merits of
various materials and sizes of materials as conductors are designated,
is called an _ohm_, (just as the unit of lineal measurement whereby
the comparative lengths and sizes of various objects are designated,
is termed a _foot_) and we will therefore use that term in reference
to the resistance of a conductor.
[Illustration: Fig. 1]
Figure 1 represents an ordinary gravity battery, the conductors from
it, and the electro-magnet to which they connect; also the armature as
attracted by the magnet and overcoming the spring which tends to
withdraw it from the magnet.
Now, as long as the current flows through the magnet, this condition
of things remains unaltered; but let a second path be presented to the
current several hundred times less in resistance than the original
one, and the result is that several hundred parts of the current will
leave the magnet for the "short circuit," and consequently leave so
little remaining in the original one that the effect will be
practically to demagnetize the magnets.
[Illustration: Fig. 2]
Figure 2 will render this very apparent if we will assume the wire of
the magnet R to possess a resistance of 10 ohms, and the conductors
themselves a resistance so low as to be inappreciable and unworthy of
consideration.
[Illustration: Fig. 3]
[Illustration: Fig. 4]
Now, assume the current to be flowing and the magnet to be charged,
and let us take a piece of metal which has an electrical resistance of
1/100 of an ohm, and lay it across the conductors at any point between
the battery and the magnet. The result is, that instead of flowing
through 10 ohms resistance _via_ the magnet, it follows the invariable
rule, and takes that offering but 1/100 of an ohm; or, more to the
point, if we assume the conductors referred to to be one mile of steel
rails each (Fig. 3), and again leave their resistance (which would be
about one ohm each) out of consideration entirely, leaving that of the
magnet as first stated, and assume the bar of 1/100 of an ohm to be an
axle and pair of wheels (_a_) of a train (Fig. 4), which possess the
same resistance, we can readily see that the result would be exactly
the same, _i.e._, instead of all the current passing through the
magnets, as when the rails were unoccupied, the presence of the wheels
upon them would cause 999/1000 of the current to leave the magnet and
pass through _them_; they offering but 999/1000 of the resistance of
the magnets, and thus leaving but 1/1000 of the whole current passing
through them, which being so small a part of so feeble a current is
imperceptible and without sufficient influence to hold the magnet
charged. Therefore, it follows that the instant that a pair of wheels
enters upon a pair of rails which thus form part of the conductors of
an electrical current holding charged a magnet, that magnet becomes
practically demagnetized, and consequently loses all power to overcome
any opposing force in its armature.
When the armature of a magnet is arranged upon a small lever, by
motion of which a second circuit is closed or opened, or two or more
circuits are otherwise controlled, the entire device is termed a
relay. In all forms of this instrument, as is the case with almost
every other electrical instrument, the armature is so arranged as to
fall by gravity, or by tension of a small spring suitably arranged,
away from the cores of the magnets when they become demagnetized.
[Illustration: Fig. 5]
When switches are included in a track section (Fig. 5), it becomes
necessary for safety to have them control the track section in such a
way that unless they are properly set (and locked, if desired) for the
main track, the continuity of the rail circuit is interrupted and the
signal is thereby held at danger. To render more certain this result,
the circuit controller (switch box) at the switch is arranged in such
a way that the track circuit is not only interrupted beyond the
switch, but is also short-circuited by it when the switch is not
properly set. It is also necessary for safety that the side track from
the switch points back to the fouling point (Fig. 5) be included in
the track section: thus insuring that all trains on these tracks are
out of danger of collision with the main track when a "clear" signal
is displayed on it.
In dividing tracks into distinct electrical sections, it becomes
necessary to insulate the rail ends at the terminal of each, from
those of the adjacent sections. If this were not done the current of
each section would traverse the next, and continue on indefinitely,
influencing each other so as to interfere with or totally prevent the
operation of all.
In order that we may fully comprehend the nature of an insulation, let
us make clear a few facts concerning conductors in general. All
materials conduct electricity to a certain extent; but some with much
more freedom than others. Thus, silver, copper, gold, zinc, platinum,
iron, steel, mercury, and other pure metals permit the passage of an
electric current through them with but slight resistance, (although
all offer a certain amount,) and are therefore termed _conductors_.
The following liquids are classed as conductors: concentrated and
diluted acids, saline liquids and water, although they are much less
efficient as such than the metals.
To this list might be added the earth itself and the various
ingredients forming it, the nature of which ingredients determines
very much its efficiency as a conductor. Thus at points abounding in
mineral deposits the earth would be far superior as a conductor to
those parts in which none exist, but at best should be regarded as a
poor conductor.
Next comes a class of materials which offer a great resistance to the
current, and which from that reason are termed non-conductors, or
insulators; of this class, rubber, glass, leather, resin, wood,
brimstone and dry air are the most common.
Wood being a non-conductor, it is very evident that the cross-ties
under the steel rails form an insulation between them and the ground;
also, that if a piece of the same or similar material be placed
between the rail ends, and that if two other pieces of sufficient
strength be substituted for the iron fish-plates at that point, a
secure insulation will be formed between the rails.
It is precisely in this way that the insulation of one rail from
another is effected (Fig. 6) and the long, practical use of many
hundred joints of this sort, has proved it to be a method both
economical and thoroughly efficient.
[Illustration: Fig. 6]
A much more secure _joint_, however, is obtained by insulating the
existing iron fish-plates from the rails by means of heavy fiber
plates, and their bolts from the rails by fiber bushings (Fig. 7).
While this method is superior to the first mentioned one in that it
makes a more secure rail joint, it is no more efficient as an
insulation.
[Illustration: Fig. 7]
One would naturally suppose that owing to the large surface of contact
existing between the rails and their connecting or fish-plates, and
from the apparent security of that contact obtained by the bolts
through them, no trouble would be experienced by the current in
passing from one rail to the other. This, however, is not the case, as
the bolts and even the plates themselves frequently become loose, even
when provided with the best of nut locks, and the rust and dirt
settling between them and the rails oftentimes increase the resistance
of a track section to a serious extent. Again, even when tightly
bolted and locked, these plates form but an imperfect contact, owing
to the scale or rust upon them. Therefore, to insure that the
resistance of a track section may be as low and as constant as
possible, we have found it absolutely necessary to connect each two
adjacent rail ends together by means of a short piece of very strong
wire (Fig. 8).
[Illustration: Fig. 8]
These wires are termed "track wires" (bond wires) and are provided
with a button-head rivet at each end, which is securely soldered
thereto, for the purpose of securing them to the rails. (Bond wires
are now attached to the rails by channel pins or are welded on.) The
connections from the rails to the battery and relay of a track section
are secured to the rails in the same manner. The battery is usually
located in a chute or well sunk in the ground at the terminal of each
section, which is provided with an elevator in which the battery is
placed and by which it may be raised and lowered at will. All wires
when placed underground are run in grooved lumber in order that they
may be secure from injury.
Even in very wet or snowy weather a single jar of gravity battery is
generally found to furnish sufficient current to properly work the
relay at the other end of any section less than three-quarters of a
mile in length; although it frequently happens on longer sections and
occasionally on those of ordinary length that two jars are necessary.
A greater number of jars is never advisable since by increasing the
intensity of the current, the liability of its leaking from one rail
to the other during wet weather is correspondingly increased, and as
this is attended with some uncertainty in the working of the relay of
the section--due to the varying intensity of the current--it should be
carefully guarded against. As two jars of gravity battery are not
sufficient to operate a signal, lock, bell or any similar instrument
with any degree of certainty, it becomes necessary to have a second
set of batteries of a greater number of jars for that purpose. The
armature of the magnet controlled by the track section is therefore
made to control a second circuit using a battery of this sort (Figs.
3, 4 and 5) and which includes the magnet of the signal mechanism. The
use of a relay on a track section is therefore necessary; and when it
becomes necessary to control two or more devices, each requiring
independent circuits, by one track, the use of a relay is
indispensable.
Track Circuit Characteristics
While the fundamental principles of the track circuit are the same
today as they were when it was originally invented by Dr. Robinson in
1872, it has been found that it is not as simple a device as was
formerly supposed to be the case and many problems have arisen which
have required and is requiring the careful study of the signal
engineers. Accordingly, it is well to present briefly some of the
track circuit characteristics as they are known today. In the
following presentation, information has been collected from many
different sources, including abstracts from papers presented on the
track circuit by Mr. A. R. Fugina, signal engineer, and Mr. J. B.
Weigle, signal inspector on the Louisville & Nashville.
There are two general classes of track circuits, direct current and
alternating current, which may be further subdivided between single or
double rail circuits. The essential feature of the track circuit is
the insulation of each section of track from the adjoining sections.
Each rail in the section is connected to the one adjoining by bond
wires, for the purpose of making a continuous conductor from one end
of the section to the other.
Rail Bonding
Under the present methods of bonding, the angle bar carries the
greater part of the current, and bond wires frequently carry as little
as 20 per cent. of it and sometimes even less. The rail resistance is
lowest with new rails, but it gradually gets higher, due to rust and
dirt formations between the angle bar and the rail. But even with new
rail, the rail resistance varies greatly at different periods and even
at different times during any twenty-four hours. This variation is
entirely due to the fact that the angle bars carry more of the current
than the bond wires, and that the bond wires under any condition are
only large enough to carry the smaller part of the current from the
battery. The lower the resistance of the bonds the less variable will
be the rail resistance.
The resistance of the angle bars increases greatly as rail resistance
increases, as a result of which the angle bars rapidly carry less of
the current.
It is not infrequent to find the rail resistance to be as high as 0.20
ohms per 1,000 ft. of track, and we have known it to run as high as
0.264 in new rail, where especial attention had been given to
obtaining as good bonding as possible. Under such conditions the angle
bar carries very little of the current, the capacity of the bond is
not sufficient to carry the current, and the net result is a failing
track circuit, which is probably attributed to bad ballast, zinc
treated ties or other causes.
The principal defect in the track circuit is that of improper bonding.
The only explanation as to why No. 8 iron wires became standard for
bonding appears to be that the bond wires were cut from this size iron
telegraph wire which was in general use at the time rails began to be
bonded. It is important to obtain better bonding to obtain a minimum
constant rail resistance.
It has been recommended that:
First--The use of galvanized wire bonds should be eliminated.
Second--Forty per cent. copper clad bond wires should be used as a
temporary expedient to replace galvanized bond wires.
Third--Except for theft and crystallization, copper bond wires would
be much more advisable.
Fourth--Larger bond wires should be used, these bonds to be at least
equal in carrying capacity to two 46-in. No. 6 solid copper or to two
No. 2, 40 per cent. copper clad wires.
Until recently it has been the general opinion of all experts on the
track circuit that the rail resistance was rather an unimportant
factor and that, as a general rule, the change in rail resistance
could be disregarded in making track circuit investigations and
calculations.
Many bad track circuit conditions have been laid to bad ballast
conditions, zinc treated ties, wet track, etc., which, if carefully
analyzed, would have shown the trouble to be due to extremely high
rail resistance. These faulty conclusions are being drawn nearly every
day.
Single rail track circuits, so called from the fact that but one rail
is insulated, are also used. Installations of this kind are made to
avoid the expense of two insulated joints or where one rail is needed
for another circuit. Such track circuits are more liable to failure
than those having both rails insulated for the reason that the
break-down of one insulated joint will extend the circuit beyond the
proper limit and cause interference of neighboring circuits or
extended shunting of the relay, due to the presence of a train beyond
the insulated joint.
A track circuit may be made to perform two separate functions in which
the direction or polarity as well as the presence of current is made
use of in the relay, provided the first or principal function actuated
by the presence or absence of current does not interfere with the
secondary function, actuated both by the presence of current and its
polarity.
Where switches occur in a track circuit, special means must be
employed to prevent short-circuiting through the switch rods and
leakage of current to the turn-out rail. The usual method is the use
of insulated switch rods with insulated joints in the leads of the
turnout and at the fouling point of the turnout. The switch points are
bonded to the stock rails to insure shunting by a pair of wheels on
any part of the track.
None of the methods employed in running track circuits through
switches show any protection against an open switch. In order to
obtain this protection a switch instrument or switch box is used. This
consists of a device with electrical contacts, the whole mounted on a
switch timber and connected to the switch point by means of a rod so
arranged that when a switch slips open or is thrown open the movement
of the rod actuates contacts which, on being closed, form a closed
path from one rail to the other through wires connecting the rails to
the contacts, thus when the contacts are closed by a switch being
opened, the same effect is produced as if a train was on the circuit,
shunting it out.
On electrically operated roads where tracks are bonded for the return
propulsion current with heavy copper bonds, no additional bond wires
are necessary.
The Track Battery
The usual form of track circuit has a primary battery at one end of
the insulated track section, with the positive terminal of the battery
connected to one rail and the negative terminal to the other, while a
relay at the other end of the section is connected to the rails in a
similar manner. Current flows from the positive side of the battery
through the one rail, the relay and the other rail back to the
battery, thus keeping the relay energized.
For d.c. track circuits, four types of cells have been used to a
greater or less extent, the gravity cell; Lalande (soda) cell; storage
cell and dry cell. The gravity cell has a voltage of about 0.8 or 0.9
volts, the resistance varying with the manner in which the cell is
maintained and averaging about 3 ohms. It will remain active for long
periods on closed circuits without appreciable polarization. Because
of this high internal resistance usually no external resistance is
necessary to be connected between it and the rail of the track. The
e.m.f. of the Lalande (soda) cell may vary from about 0.67 volts to
0.88 volts while the internal resistance will range between 0.019
ohm to 0.4 ohm. Because of the low internal resistance of these cells
it is necessary to use an external resistance of the proper value
between the cell and the rail. The storage cell is made in various
capacities and a fully charged cell on open circuit has a voltage of
approximately 2.1 volts which, when placed on discharge, becomes
approximately 2 volts and drops to about 1.8 volts when completely
discharged. The voltage in this type of cell varies with the density
of the electrolyte and to a certain extent with temperature. It has
practically a negligible internal resistance and it is also necessary
to use an external resistance in the leads between the cell and the
track to prevent a flow of excessive current when a train occupies the
track. The dry cell is used only in emergency cases or occasionally
for open circuit track circuits of 2 or 3 rail lengths, which are
sometimes used as annunciator starts to announce the approach of a
train to a tower-man. It is designed primarily for open circuit work
and will polarize when current beyond a certain figure is drawn
continuously from it.
The Track Relay
The track relay is a development of the instrument of the same name
used in telegraph service. It consists of an electro-magnet of the
horseshoe type with a pivoted armature, carrying one or more fingers
for making or breaking electric circuits for the control of signal
apparatus.
Track relays with resistances of 2 and 4 ohms are usually employed.
From experience with two-ohm relays on the L. & N., covering a great
many of them on all kinds of circuits, the following conclusions are
reached:
The two-ohm relay is more suitable for general use on track
circuits than the four-ohm, provided not less than the R.S.A.
recommended limiting resistance is used between the battery and
track.
The two-ohm relay will operate satisfactorily where the four-ohm
will not on bad track circuits, and with considerably less current
consumption.
The two-ohm relay will operate equally as well on good track
circuits of average length as the four-ohm, there being little
difference in current consumption on this class of circuit. Under
the same conditions longer track circuits may be operated with the
two-ohm relay.
The two-ohm relay is at least as safe as the four ohm. It should be
thoroughly understood that it is as important with the four-ohm relay
as it is with the two-ohm relay to have not less than the R.S.A.
recommended limiting resistance between the battery and track. This is
important with any kind of low internal resistance battery, and under
certain conditions with gravity battery also.
In one case assume a train to be passing from the relay end to the
battery end of a track section and in the other case from the battery
end to the relay end. The effect accomplished is the same except that
the relay will not release so quickly when the train passes from the
battery end towards the relay end, and this is in part due to the
self-induction of the circuit through the relay coils, the rails and
the axles of the train. It is due more, however, to small current
leakage from the adjacent section and the effects of stray currents
which are always present to a greater or less degree. A broken rail
will also generally open the circuit and de-energize the relay.
Circuits for the control of the various signal devices are broken
through the contact points of the track relay.
Track Circuit Maintenance
Cross ties have a relatively high resistance to the passage of
electric current, but when a large number connect the rails many
multiple paths are introduced into the circuit through which the
current may flow from one rail to the other, and, considering them as
a whole, the resistance they offer to the passage of the current
reaches a relatively low value. Consequently there is always a current
leakage from rail to rail through the cross ties and ballast. Every
effort should be made to secure and maintain the best ballast and
drainage possible on d.c. as well as a.c. track circuits. Cinders,
dirty sand, soft water-logged ties and ballast not well cleaned away
from the base of the rail will produce track circuit trouble,
particularly during wet weather, while good rock ballast, sound ties
and clean track give the greatest efficiency.
The use of ties freshly treated with zinc chloride also reduces the
ballast resistance. If too many such ties are used in a track circuit
the current leakage between rails becomes so great that not enough
current reaches the relay to hold it closed, the effect being the same
as if a train is on the track circuit shunting out the relay. For good
results, the number of zinc-treated ties installed per year in any
track circuit should not be greater than 15 per cent. of the total
number of ties in that circuit.
Track Circuit Troubles
Some of the common track circuit ailments are relay and track battery
troubles, defective track connections, poor bonding and broken rails,
short circuits or shunts, excessive leakage and defective insulated
joints, all of which will cause the signals to be set in the danger
position, while defective relays, foreign current and poor wheel
contact may result in a false proceed signal indication with a train
in the block section.
It was the quite general practice to operate bad track circuits by
piling on gravity battery, either in multiple or multiple-series
arrangements to obtain operating results without any regard to the
safety of the circuit and, no doubt, many false proceed failures were
caused thereby.
The effect of temperature changes on track circuit operation are of
considerable importance. The track relay, which is generally housed in
a cast or sheet iron box, probably is affected more by changes in
temperature than any other part of the track circuit. The resistance
of a 2-ohm relay, which is 2 ohm at 70 degrees F., will be 2.22 ohm at
120 degrees F., and 1.69 ohm at 0 degrees F., a variation of .53 ohm.
The pick up and release of the relay, .2 and .1 volt, respectively, at
70 degrees F., will be .22 and .11 volt at 120 degrees F., and .17 and
.085 volt at 0 degrees F. A relay, with a normal resistance of 4 ohm
at 70 degrees F., will be 4.45 ohm at 120 degrees F. and 3.38 ohm at 0
degrees F., a variation of 1.07 ohm. The pick up and release, .3 and
.14 volt, respectively, at 70 degrees F., will be .33 and .16 volt at
120 degrees F. and .25 and .12 volt at zero.
The point which is intended to be brought out by these figures is that
when the temperature of the relay increases, a correspondingly higher
voltage is required to pick up the armature, and when the temperature
decreases the armature will hold up with lower voltage across the
coils. This indicates that a track relay is more liable to fail to
release due to an imperfect train shunt in cold weather than at any
other time.
Some of the best preventatives that may be provided to guard against
false proceed signals due to track relays failing to release with a
train in the circuit, are:
1. Use as much resistance as practicable between battery and
track.
2. Use low resistance bond wires, and maintain bonding in good
shape.
3. Keep ballast well cleared from contact with rails.
4. Maintain insulation in insulated track joints in good condition.
Aside from these simple remedies no definite rule can be given to
combat foreign current. If it is so troublesome that these methods do
not overcome it, the circuit affected must be carefully studied to
determine the source of the foreign current and its path to the rails,
then special means can usually be provided to overcome it.
Ballast Resistance and Leakage
The importance of ballast resistance has long been recognized, and
this always has been considered the great variable, whereas,
investigations show that the ballast resistance is at least no more
variable than the rail resistance, and that of the two it is more
important to reduce the rail resistance to a minimum, and especially
to establish it as a constant.
When the ballast leakage problem was first taken up (on the L. & N.),
various kinds of ballast were measured in both wet and dry weather,
the intention being to determine the lowest possible resistance per
1000 ft. for each kind of ballast. It was proposed in this way to
establish a standard minimum resistance per 1000 ft. for each kind of
ballast. For instance, if a number of measurements in wet weather
showed 8 ohms per 1000 ft. as a minimum for track circuits with
crushed rock ballast, it was the intention to adopt 8 ohms as the
standard minimum ballast resistance per 1000 ft. for all track
circuits where crushed rock ballast was in use. If a number of wet
weather measurements showed 4 ohms per 1000 ft. as a minimum for
cinder ballast, it was the intention to adopt 4 ohms as the standard
minimum ballast resistance per 1000 ft. for all track circuits where
cinder ballast was used. It was the intention to follow out the same
process and establish a standard for all kinds of ballast in use. This
was soon found to be impracticable.
After making many ballast resistance measurements, it was noticed that
the variation of the resistance on any track circuit, as between wet
and dry weather, generally followed quite a definite rule. For
instance: If the resistance per 1000 ft. of dry ballast was found to
be 28 ohms or more, it would be not less than 8 ohms per 1000 ft. when
wet; or if resistance of dry ballast was found to be between 22 and 28
ohms per 1000 ft., it would be not less than 6 ohms per 1000 ft. when
wet.
Once a relay is picked up or energized, but a small amount of current
is required to maintain it in that condition. This is one reason why
it is important to keep the ballast clear of the rails and it is
because of the condition which may cause a relay to remain energized
that rules are in force requiring the signalmen to disconnect a track
relay when track forces are changing out rails.
Combined Rail and Bond Wire Resistance
On circuits newly bonded with two 46-in. galvanized iron wires a
joint, the combined rail and bond wire resistance was found (on the L.
& N.) to vary from .02 ohm per 1000 ft. of track on some circuits to
.265 ohm on others, a difference of over 1300 per cent. This was
rather puzzling. After a great many measurements had been made on
different circuits it was found that no two measurements gave the same
results, notwithstanding the fact that in many circuits the size of
rail, length of bond wires, and age of bonding were exactly the same.
On account of the bonding being new and the channel pins well driven,
the contact between the bond wire and rail was above suspicion. The
only other part of a track circuit that could possibly be the cause of
this difference was in the contact between the angle bars and rails,
and this later proved to be the case. Actual measurements made in the
field proved that when the rail is new and the joint bolts tight,
nearly all of the current flowing from rail to rail passes through the
angle bars, whereas when the rails get old a coating of rust and dirt
forms between the rail and angle bars, forcing practically all of the
current through the bond wires. On most of the circuits measured on
the L. & N. the combined rail and bond wire resistance was found to be
less than .1 ohm per 1000 ft. of track, although many were found to be
between .10 and .30 ohm per 1000 ft. It is interesting to note that
two circuits were found bonded with two 52-in. iron wires, for which
the combined rail and bond wire resistance measured .410 ohm, and that
by adding two 40 per cent. copper clad bond wires to each joint the
combined resistance was reduced to .144 ohm.
The Growth of the Track Circuit
Unfortunately there exists little or no data regarding the mileage of
track circuits installed from the time the first installation was made
by Dr. Robinson at Kinzua, Pa., and Irvineton up to about 1905. During
the period between January 1, 1905, and September 30, 1906, the total
automatic block signal mileage installed was 1,710.6, which brought
the total up to 6,826.9 for the United States. Between September 30,
1906, and January 1, 1908, 3,976.1 miles of automatic signals were
installed, which increased the above total to 10,803.0 miles.
The Block Signal and Train Control Board, seeing the need for accurate
data in the signal field, started the tabulation of such statistics
when it compiled and issued Block Signal Statistics as of January 1,
1908. After this board went out of existence, the Bureau of Safety of
the Interstate Commerce Commission continued the collection and
publication of these data yearly. Perhaps no better word picture can
be given of what Dr. Robinson's invention has meant to the railroads
than to present the story in the form of a table showing the miles of
road and the track equipped with the track circuit since January 1,
1908. In addition to the table, the accompanying chart presents the
information in a graphical form.
[Illustration: Progress Chart of Automatic Signal Installations
Since January 1, 1908.]
Track Circuit Mileage for Automatic and Controlled Manual Signals
in the United States as Taken from I.C.C. Reports
----------+-------------------+---------------------------------------
| Automatic | Controlled Manual
+---------+---------+-------------------+-------------------
| | | Miles of Road | Miles of Track
| Miles | Miles +--------+----------+--------+----------
| of | of | Track |Continuous| Track |Continuous
| Road | Track |Circuits| Track |Circuits| Track
| | | at | Circuits | at | Circuits
| | |Station | |Station |
----------+---------+---------+--------+----------+--------+----------
January 1,|
1908 | 10,819.3| 18,534.1| 726.7 | 212.0 | 2118.0 | 410.8
1909 | 12,174.3| 20,590.9| 407.6 | 572.2 | 978.0 | 1413.0
1910 | 14,238.9| 23,771.3| 385.8 | 491.5 | 953.5 | 1371.3
1911 | 17,709.8| 29,151.6| 483.9 | 439.4 | 1119.3 | 739.9
1912 | 20,300.0| 33,343.8| 402.0 | 295.9 | 955.6 | 496.0
1913 | 22,196.6| 36,873.0| 370.2 | 228.3 | 868.9 | 380.3
1914 | 26,569.3| 44,461.2| 275.7 | 180.3 | 625.3 | 281.6
1915 | 29,863.5| 49,442.1| 250.5 | 145.1 | 549.3 | 185.7
1916 | 30,942.5| 51,119.7| 255.1 | 125.1 | 549.6 | 179.3
1917 | 32,954.6| 53,799.8| 230.3 | 132.0 | 524.8 | 155.5
1918 | 35,193.1| 57,083.6| 208.1 | 131.2 | 451.6 | 154.8
1919 | 36,989.4| 59,458.2| 221.2 | 256.9 | 483.8 | 441.2
1920 | 37,968.8| 60,992.3| 196.3 | 129.2 | 413.3 | 151.4
1921 | 38,543.9| 61,744.5| 206.8 | 125.7 | 422.6 | 166.4
----------+---------+---------+--------+----------+--------+----------
The first yearly report of the Bureau of Safety, I.C.C., on block
signals to contain information as to the miles of road and miles of
track on which alternating current track circuits were installed, was
that issued as of January 1, 1914. Data taken from that report up to
the last one issued is presented in the table below.
Alternating Current Track Circuit Mileage
----------------+---------------+----------------
| Miles of Road | Miles of Track
----------------+---------------+----------------
January 1, 1914 | 3,289.2 | 4,144.6
January 1, 1915 | 2,728.2 | 5,814.9
January 1, 1916 | 3,186.7 | 6,679.0
January 1, 1917 | 3,336.2 | 6,823.6
January 1, 1918 | 3,748.0 | 7,530.1
January 1, 1919 | 4,496.6 | 8,620.2
January 1, 1920 | 4,676.5 | 9,026.0
January 1, 1921 | 4,786.1 | 9,120.2
----------------+---------------+----------------
Alternating current track circuits have certain advantages over direct
current track circuits, particularly in respect to their immunity to
the dangerous effects of foreign direct current to which d.c. track
circuits in some communities are subjected. The above table is
therefore of interest as it shows the application of alternating
current as made to Dr. Robinson's invention of the closed track
circuit.