_Answer._ The truck frame is extended some distance behind the truck-axle, as shown in fig. 221, and the centre-pin, _a_, about which it vibrates, is placed at the back end. The weight of the locomotive, or that portion to be carried on the truck, is then made to rest over the centre of the axle, but in such a way that it can move laterally or crosswise over the track. Such trucks were first made so that the weight of the engine rested on slides on the truck frame, but recently they are nearly always suspended on links, so that they can swing like a pendulum, as shown in figs. 190 and 191. The weight of the engine then rests on the centre-plate, _H H_; which forms part of the plate, _B B_. This is suspended by links, _L L_, represented by dotted lines, which are attached by bolts to the cross-pieces, _m m_, which are fastened to the truck frame. In this way the truck-wheels can move sideways independent of the engine itself. As the wheels and axles, _A A_, must move about the centre-pin, _a_, fig. 221, the axle assumes a radial position to the curves of the track. It does this, too, quite independent of the driving-wheels, as is shown in fig. 221, which represents a plan of the wheels on a curve. It will be seen that the truck is not at all influenced by the position of the driving-wheels. This arrangement therefore gives great flexibility to the wheel-base, and enables the wheels to adjust themselves to any lateral curvature or alignment of the track. [Illustration: Fig. 221. Scale ¹⁄₈ in. = 1 foot.] QUESTION 435. _For what purpose are locomotives like that shown in Plates XI and XII used?_ _Answer._ “Mogul” locomotives are often used for ordinary freight service where heavy trains must be hauled, and also on steep grades. “Consolidation” locomotives, represented in Plate XII, which have eight driving-wheels, are employed almost exclusively for traffic over heavy mountain grades. QUESTION 436. _What other kinds of locomotives are used for freight traffic?_