Fig. 446 is a sketch of a carriage-traverser, of length to suit an ordinary six-wheeled carriage. The length, however, may be extended to take on a bogie carriage or any other long carriage. The framing is made of wrought-iron or steel, well braced together. The carrying wheels, W, W, run upon rails laid at right angles to the running-line or siding, and the carriage is moved on to or off the traverser by means of the hinged ramps shown at R, R. A carriage, once on the traverser, may be moved across one or several lines of running road, according to the extent of traverser line laid down; and this appliance is very suitable for large terminal stations and carriage-repair shops. It will be observed that the operations of the 294 turn-table and the traverser are quite distinct. With the former a vehicle can be transferred from one line to another, and also turned completely round; but with the traverser the vehicles are simply moved in a parallel direction, from one line to another, and when it is necessary to turn or change a vehicle end for end, as in the case of a mail-bag-catching apparatus van or a special saloon, then resort must be had to a turn-table. Cranes.--A large portion of the merchandise conveyed on railways must be lifted into or out of the trucks by cranes. The position, description, and capacity of these will depend upon the materials to be handled. Large slow-working powerful cranes will be necessary for raising heavy castings, large logs of timber, or massive blocks of stone; while the small quick-acting cranes will be more suitable for dealing with the lighter packages, casks, and bales. Fig. 447 shows a gantry or overhead crane, used for lifting heavy weights out of an ordinary road-waggon, carrying them a short distance, and then depositing them in a railway truck, or _vice versâ_. Double-flanged rollers, attached to the ends of the platform C, C, run upon the rails R, R, which are fixed on the top of the beams B, B, secured to the verticals A, A. The working length of the gantry is only limited by the number of the verticals, and this, being the fixed portion of the work, may be extended out to any distance required. The travelling or carrying girders of the platform C, C may be made of wrought-iron, steel, or timber. They must be strongly framed and braced together as a platform to carry the lifting machinery and weight lifted, and have convenient gearing for effecting the transverse or side-to-side movement, as well as a horizontal movement along the line of rails on top of the verticals. Where the fixed portion of the gantry is of considerable length, two or more travelling platforms can be used. In the sketch given above, the entire gantry is shown as made of timber, but iron or steel can be equally well adopted, and continuous masonry or brickwork walls may be built to serve as verticals. [Illustration: Fig. 448, 449, 450]