QUESTION 249. _What proportion of the weight of ordinary locomotives is usually carried on the driving wheels?_ _Answer._ Eight-wheeled “American” locomotives, which are most commonly used in this country, have about two-thirds of their weight on the driving wheels. QUESTION 250. _What is meant by the “truck” of a locomotive?_ _Answer._ It means one or more pairs of wheels which are attached to a separate frame and to the locomotive by a flexible connection, so that the axles are not held rigidly at right angles to the main frame, but can assume positions which approximate to that of radii of the curves of the track. In plates I, II and III, _E E_ are the truck wheels, _b′ b′_ the truck frame, and _y_, plate II, and fig. 40, the centre-pin, around which the truck frame turns. QUESTION 251. _What service does the truck perform?_ _Answer._ It carries the weight of the front end of the locomotive, and also guides it into and around curves and _switches_.[67] [67] A switch is a movable pair of rails, by which a locomotive is enabled to run from one track to another. QUESTION 252. _How does it perform the latter service?_ _Answer._ It does it very much in the same way as the front wheels of an ordinary wagon enable it to turn around corners; that is, the truck wheels being attached to a separate frame, which is connected to the locomotive by a centre-pin, just as the front axle of an ordinary wagon is connected by the king-bolt, can turn. [Illustration: _Fig. 149._ _Fig. 150._ Scale, ³⁄₈ inch = 1 foot.] [Illustration: _Fig. 151._] QUESTION 253. _Why are two pairs of wheels used on a locomotive instead of one, as on an ordinary wagon?_