With respect to the “level” at which the line should be laid, a
section of the route, showing all the elevations and depressions, is
first made, then such a course is chosen that the material produced
by cutting through the higher parts shall be just sufficient to form
the embankments for filling up the lower parts; fig. 1 will give
some idea of this arrangement. Of course a line perfectly level
would be the best, just as would be one perfectly straight; but as
the difficulties of the one must be balanced, so must those of the
other, and a line as nearly level should be obtained as is consistent
with expense. For instance, suppose A and B, fig. 2, to be towns to
be connected by a line of railway, and the chief of the intermediate
ground to be above their level; of course it would be very expensive
to cut through the whole distance, as shown at the dotted line
_a_, this level would therefore be too low; but if a higher level
were taken, as at the dotted line _b_, then only the centre of the
distance would have to be cut through, and the material (earth, &c.)
produced by the cuttings would suffice to fill up the hollows at the
ends. These considerations and many others must therefore determine
the level at which the railway shall be constructed; but the line is
seldom (if ever) on one level from end to end, nor at one continuous
“gradient” or slope, for the course of the line is so arranged as to
make as little cutting and filling up as is consistent with a road
whose gradients shall never exceed a specified amount, which must
be determined by local circumstances. The excavation and filling
up being finished, the “trams” have to be laid; these are bars of
wrought iron about fifteen feet long, of the form shown at A and
B, fig. 3. The most usual form is that marked A. They are made of
wrought-iron, passed while hot between rollers cut at their edges
into the form required. These trams are laid upon bars of wood called
“sleepers,” at about four feet apart, and united to them by what are
called “chairs,” which are pieces of cast-iron of the form shown at
fig. 4, fastened to the sleepers by iron spikes, and into these the
trams, or “metals,” as they are called by the workmen are wedged.
These bars of iron are laid very evenly and perfectly parallel at a
certain distance apart, which must exactly correspond to the distance
between each wheel of a pair belonging to the carriages and engines;
this distance is called the “gauge,” the wide gauge (as on the Great
Western) is seven feet, and that called the “narrow gauge,” is four
feet eight-and-a-half inches, and the space between the lines is of
sufficient width to prevent any danger of collision in the trains on
passing each other; they are generally six feet apart.