ELECTRIC RAILWAYS FROM THE ENGINEERING POINT OF VIEW
When electric railways were first considered, the natural tendency
of engineers was to follow the existing model and merely substitute
electric locomotives for steam locomotives. In point of fact, however,
the engineering method now adopted is an evolution from the tramway
model, not from that of the typical railway.
A certain advantage was, of course, to be gained by replacing steam
locomotives by electric ones. The greater 'starting torque' of the
electric locomotive enables it to get a train up to full speed more
quickly; and the capacity of the electric motor for taking heavy
overloads assists the electric train in surmounting heavy gradients.
Some advantage was also gained by producing all the power at a central
source, instead of having a large number of steam locomotives, which
are really power stations on wheels. But the electric locomotive had
still to be made heavy enough to get sufficient grip of the rails;
it had to haul its own dead weight; and it had to be made powerful
enough to tackle a full-sized train on the steepest gradient with its
complement of passengers, although the general demand upon it might be
considerably less than that maximum.
The electric locomotive, in short, was an advance upon the steam
locomotive, but it did not get past the essential drawbacks of the
locomotive system. A locomotive is most economical when hauling full
trains for long distances at a uniform speed; it is essentially a
long-distance machine. The first demand for electrification came,
however, from suburban railways, where the stations are close together
and where, therefore, the speed is constantly varying from zero up to
a maximum and back to zero again. The traffic also fluctuates between
extreme limits; and there is obvious waste in having to run heavy
locomotives and trains backwards and forwards during the slack hours.
There was therefore a demand for some method of propulsion which would
enable the length of trains and the consumption of power to be adjusted
more closely to the variations in the traffic.
A step in the right direction was taken when the locomotive equipment
was placed on a car, thus utilising the weight of the passengers to
increase the adhesion on the rails. But the full advantages of electric
traction were not realised until what is known as the 'multiple-unit'
system was adopted.
The idea underlying this system is quite simple. If, instead of
concentrating the motive power on a single locomotive or driving
unit, we distribute it among the cars forming a train, we get the
multiple-unit system. An electric tramcar and a trailer attached
to another tramcar and trailer, with a third tramcar behind, would
form a model for a multiple-unit train. By connecting the electrical
equipments on the three tramcars--front, middle, and rear--it would be
possible to control the train from either end or from the middle.
This is the principle upon which all the electric railways in Great
Britain are now worked, with the exception of the City and South London
Railway, where locomotives are still used and where the trains are
comparatively short and light.
It will be seen that each multiple-unit train is readily divisible. A
single motor car may be run, or a car with one or two trailers, or a
long train made up of as many motor cars and trailers as the platforms
will accommodate. And whether the trains are long or short, the power
absorbed is in proportion to the length of the train and the load of
passengers. By this simple means power is economised, and the railway
engineer is able to reduce the proportion of idle rolling stock.
The adjustment of the length of trains to the fluctuations of the
service is made easier by the absence, in the multiple-unit system,
of the necessity of shunting at the termini. As a multiple-unit train
can be controlled from either end, a more frequent as well as a more
flexible service can be run. With steam traction the number of trains
which may enter or leave a terminus is limited by the time occupied in
shunting and by the necessity of leaving lines of rails free for that
operation. With an electric train on the multiple-unit system, no more
time is lost than the few seconds necessary for the driver to walk from
the front of the train to the rear, which then becomes the 'front.'
No lines have to be kept open for shunting locomotives, so that the
available accommodation for trains is considerably increased. Some of
the London railway companies have spent enormous sums in enlarging
their terminal accommodation and have found that it is still inadequate
to the demands of the 'rush' traffic. Electric traction therefore
offers them an improvement of enormous value without the expenditure of
a penny on station alterations.
The crowning advantage of electric traction lies, however, in the more
rapid acceleration which it affords. We have already seen how important
this item is on tramways. It is still more important on suburban
railways, where a high average speed, in spite of frequent stops, is a
vital matter.
On the District Railway the rate of acceleration in the old steam
days was about 6 inches per second per second. It was, in fact, so
low that the trains could not reach a fair speed before the brakes
had to be applied to bring the train to a stop at the next station.
With electric traction the rate of acceleration has risen to about
18 inches per second per second. On the Liverpool Overhead Railway a
rate of 36 inches per second per second was reached in certain tests.
Heavy starting currents are, of course, necessary to bring a train from
rest to full speed at such a rapid rate, but it is quite possible for
the electrical engineer, without being unduly extravagant in current,
to accelerate a train more quickly than the passengers would find
comfortable.
The practical result of rapid acceleration (combined with rapid
braking) is not only to give a higher average speed but also to enable
a more frequent service to be run. Owing to the block system on
railways it is impossible for trains to follow each other closely in
the manner of tramcars; and it is therefore of cardinal importance that
no train should occupy a block for one second more than is necessary.
Rapid acceleration becomes all the more important in this respect
because of the difficulty of setting down and picking up passengers
quickly. This difficulty is overcome in part by using saloon carriages
with middle and end doors, in place of compartment carriages. At first
the District Railway tried to help matters by operating these doors
pneumatically, but the mechanism became unpopular after a number of
late-comers had been pinched by closing doors. The management has
reverted to hand operation; and it has probably achieved more by
educating the public to move quickly than it would have gained with its
too-perfect mechanical system.
London travellers have become so accustomed to entering and leaving
trains quickly that it is possible for an observer to distinguish
strangers by their slower movements on an underground railway. Thus
the passenger, as well as the service, has been 'speeded-up.' The more
frequent service of trains with a higher average speed would not have
been possible, however, without an improvement upon the old methods of
signalling. There is no need to dwell upon the weakness of the human
element in railway signalling; and it will be clear even to the layman
that the strain of handling traffic with a headway of one minute and a
half, or less, would be more than men could stand. Automatic signalling
had therefore to be adopted to obviate the risk of disaster.
Each train, as it leaves a block or section, 'clears' the signals
for that block; and when any train attempts to enter a block against
signals, the current is automatically switched off and the brakes
applied. The system is so perfect that, in spite of the enormous
traffic worked under it, there has been no failure and no accident.
It is, of course, costly to install; and its cost can be justified
(financially) only when the traffic is very heavy--that is to say, when
the conditions make it almost a necessity.
The supply of electric power to electric railways is organised on
practically the same lines as in the case of tramways. That is to say,
current is generated at a central station, transmitted at high pressure
to various sub-stations, and supplied from there at working pressure
through 'feeders' to each section of the system. In the case of the
'Underground' system, most of the power is taken from a single huge
electric station at Chelsea. Current from that station drives trains as
far west as Wimbledon, Hounslow, and Ealing, as far north as Highgate
and Golder's Green, and as far east as Barking.
This is a magnificent example of the concentration which gives
economy. If each of the underground railways forming the system had
erected its own generating station, the total initial outlay, on
land, buildings, and machinery, would have been greater, and the
cost of current would have been higher, owing to the smaller output
and the more irregular demand which a single railway affords. The
ideal electric power station is one which is constructed with the
largest generating units and produces current at its maximum capacity
throughout the twenty-four hours of each day. The Chelsea power station
is nearer the ideal than a smaller one supplying a short railway could
be. And a station of the latter class is, it may be noted, nearer the
ideal than the arrangements on a steam railway, where the sources of
power are scattered in hundreds of locomotives.
The concentration of power is therefore one of the many factors which
have enabled electric railways to give a vastly improved service at
lower fares.
With two exceptions--to be considered in the next chapter--the electric
railways of Great Britain are constructed on the 'third-rail' system.
They are thus a reversion to--or, rather, a survival of--the original
type adopted by Siemens in 1879. The 'third-rail' is carried on
insulators a few inches outside the track rail; and the motor cars are
provided with a 'brush' or 'shoe' which slides along it and collects
the current. In the centre of the track there is generally a second
insulated rail to carry the return current, as it is more convenient,
under railway conditions, to have a conductor independent of the track
rails than to follow the tramway plan of using the rails 'bonded'
together. In stations and at crossings the third or 'live' rail is
protected by a wooden board in order to reduce the risk of shock to
anyone falling on the line or walking upon it. The board is placed high
enough over the rail to allow the shoe to pass freely.
As regards the motor equipment on the cars, tramway models have been
followed very closely. The 'series-parallel' system of control is again
adopted in order to get the high starting torque which gives rapid
acceleration with moderate current consumption. The course of the
current is again from the live rail, through the controller, through
the motors, and thence to the return rail. The controller itself is
more or less on the tramway principle; and the main modification in
it is the arrangement which enables all the motors on a multiple-unit
train to be operated by a single controller. This is done by connecting
the controllers electrically and using electric power so that they all
work in unison. Some companies use, for this purpose, compressed air
controlled by electricity instead of electric power alone, but in both
cases the principle is essentially the same.
Considered as a whole, the difference between a tramway and an electric
railway on the third-rail system is a difference in degree, not in
kind. The traffic is greater and the speeds higher, but both serve
the purposes of comparatively short-distance transit. Indeed, within
certain limits they compete with each other.
There remains to be considered another type of British electric
railway which points the way to the extension of the new mode of
traction to main line railways.