There are many designs of marshalling yards such as might be required at M, but there are three main types: (1) the flat shunt ing yard, (2) the hump shunting yard, (3) the gravity shunting yard. The choice of design will rest upon the number of wagons to be dealt with in hours of greatest traffic, and also upon the physical characteristics of the ground which is available for use as a marshalling yard. In the ,:ase of a flat yard the train enters direct upon one of the sidings, and a shunting engine draws it out and proceeds to push the wagons, in "cuts" of one or more wagons at a time into the lines upon which trains for the varying destinations are being assembled. Flat shunting is in reality a pull and push system and bears hard on the couplings, on the engine brakes and on the rails. Where more wagons per hour must be sorted the train is usually left by the train engine on a recep tion siding outside the main "grid" of sorting sidings, and a powerful engine is attached to the rear of the train, which is then pushed over a "hump," or high shoulder of ground, and the cuts for each line are uncoupled as the wagons reach the crest of the hump, when the couplings are loose. The wagons
thus released drop down by gravity on to the sorting sidings arranged in fan design from the bottom of the hump. One such hump line may serve 4o sorting sidings. An alternative system is to place the reception sidings upon a downward gradient lead ing to the sorting sidings, so that after the train engine has been uncoupled the wagons drop down by gravity themselves on to the various sorting sidings.
Wagons may be braked by hand or by means of rail brakes, or car retarders, operated by pneumatic or electrical power. These consist of rails placed inside the running rail so as to grip the flanges on the inside of the wheel. The Frolich system of rail brakes, as used in Germany, utilizes the weight of the wagon for braking purposes. In continuation of the sorting sidings, where possible it is desirable to have a few so-called departure sidings upon which the wagons on a train may be remarshalled into the order of the stations for which the wagons are destined. The marshalling yard is a nodal point in railway operating organization, and its working must be very carefully supervised.