An arriving goods train is first drawn out of the main-traffic lines into one of the shunting lines, and then handed over to the staff of men in charge of the sorting operations, who at once mark the waggons according to the number or designation of the particular siding into which they have to be placed. A suitable engine is generally set apart for this work, and in a very short time the entire train is divided out by one or more waggons at a time, and distributed into the various sidings, representing different sections of the line, or groups to be handed over to neighbouring railways. When one of these sorting-sidings contains a full complement of waggons, an engine is attached, and the train despatched to its destination, leaving the siding clear for another set of waggons. Where the trains to be sorted are very numerous, two or more shunting-engines may be engaged working at the same time on distinct sets of shunting lines and sidings. Sometimes it may be expedient to have one lot of sorting-sidings leading off the UP line, and another lot leading off the DOWN line, to meet the requirements of trains coming and going in both directions. With sidings well laid out, and fitted with ample facilities, a well-organized staff can carry out a very large amount of work both expeditiously and economically. There are several of these sorting-sidings stations in operation, where from one thousand to two thousand waggons are sorted and marshalled into trains every twenty-four hours. The above diagram sketches merely illustrate the general principle of the sorting-sidings, and may be modified and enlarged in many ways to suit the traffic requirements and local surroundings. Turn-tables.--Turn-tables revolving on fixed centres are made of various sizes according to their use for engines, carriages, or waggons. The carrying-beams may be made of cast-iron, wrought-iron, or steel, but the latter material is the most suitable for tables of more than 20 feet diameter. For small turn-tables, cast-iron beams will serve very well, for although more liable to fracture, they will not suffer so much from rust and oxidization as wrought-iron or steel.288 [Illustration: Fig. 436, 437, 438, 439, 440]