Sorting-sidings.--On many important long main lines it is necessary
to establish special independent sidings for sorting or arranging
waggons of merchandise and minerals. Where there are only two lines of
rails to serve for the UP and DOWN service of a heavy passenger and
goods traffic, it is imperative to restrict those lines as much as
possible to the actual transit of trains, and not to block them by
unnecessary occupation for shunting purposes. A goods train running a
long distance collects waggons from many roadside stations, and at
some of them several waggons will be taken on, to be forwarded to
various and widely distant destinations. The accumulated train
comprises waggons which must be divided out into groups, to be passed
on either to distant sections of the same railway system, or on to
neighbouring lines. To avoid interruption to the train-working, and
the delay of complicated shunting operations at the roadside stations,
the waggons are attached just as they are dicked up, and the work of
sorting is allowed to stand over until the train arrives at the place
assigned for the purpose. A site for sorting-sidings is generally
selected where the ground and gradient are favourable, and where ample
room can be obtained for a large number of short parallel lines, with
space for future extensions. The arrangement that naturally suggests
itself is that of a series of fan-shaped sidings leading out of main
shunting lines, separate from the main-traffic lines. In some cases
the sorting-sidings are laid down with dead-ends, as in Fig. 434, and
in others they are made as through sidings, connecting at both ends
with shunting lines and main-traffic lines, as in Fig. 435. Each of
the sidings is usually made sufficiently long to hold a complete train
of sorted waggons, and the number of them will depend upon the number 287
of sections to be served, and the amount of waggons to be sorted.
Sometimes the sidings are laid with a slight falling gradient leading
away from the main shunting lines, to facilitate the running out of
the waggons into the respective sidings.
[Illustration: Fig. 434, 435]