The form or contour line of the top ballast will vary
according to circumstances. In station-yards it is usual to fill in the ballast almost up to the level of the top of the rails for the convenience and safety of the men who are constantly moving about marshalling the carriages and waggons. Out on the open line between stations, the ballast on some railways is filled in up to rail-level, while on others it is only filled in up to the tops of the sleepers, leaving the rails and chairs quite clear of the ballast. On others, again, the ballast is filled well up to the rails and channelled in the centre, as shown on the sketches [Figs. 336 and 337]. Channelling the centre of the road reduces the quantity of ballast per mile, ensures good drainage, and also stability by not permitting any central support to the sleepers. By covering up the lower table and sides of rails the noise is reduced to a minimum, vibration is absorbed, and a more silent road is the result. The contact with the ballast also preserves the rail from the extremes of temperature. Where the ballasting is not channelled there is some risk of the sleepers breaking in the middle. The constant packing of the sleepers just under the rails has a tendency to drift some of the ballast inwards towards the middle of the sleeper, forming a hard compact mass, and this mass, acting as fulcrum, throws considerable strain on the middle of the sleeper when the trains pass over and depress the ends. Where the ballast is filled in level with the rails on top of sleepers it should be loosened occasionally in the middle to prevent it becoming too hard.
Connections with the rails of the main line will have to be made in various forms to suit the circumstances of the joining lines or sidings.
[Fig. 345] shows a simple double-line junction.
[Fig. 346] shows an example of what is termed a flying junction, or a junction of two double lines arranged in such a manner as to cause the least interruption to a constant train traffic passing UP and DOWN over both lines. Upon referring to [Fig. 345] it will be seen that a train from F, turning off at the points E and proceeding to G, must block, or close for traffic the section ABC during its passage over that line towards G. With a crowded train-service the blocking of both UP and DOWN main lines for the working of one train would cause much interruption, and to obviate such delay the flying junction is substituted. [Fig. 346] shows how a train from F is turned off at the points J and proceeds on to K, where by means of a bridge
it passes either over or under both main lines, and continues on to G without in any way interfering with the train service on ABC.
[Fig. 347] is an ordinary plain siding or turn-out, including the necessary throw-off or trap-points and short dead end.
[Fig. 348] is an ordinary cross-over road from DOWN main line to UP main line, and vice versâ.