trailing wheels first appeared on american locomotives between 1890 and 1895, but their axle worked in rigid pedestals. it enabled boilers to be lowered, since the top of the main frames was dropped down behind the driving wheels and under the firebox. the firebox could also be longer and wider, increasing the heating surface area and steam generation capacity of the boiler, and therefore its power. the concept was soon improved to provide radial lateral movement by placing the pair of trailing wheels and their axle in a fabricated sub-frame or truck, usually with outside bearings as they gave the best lateral riding stability. one-piece cast-steel trailer trucks were developed about 1915, to provide the additional strength for a booster engine to be fitted to the trailing axle. finally, about 1921 the delta trailing truck was developed with an inverted-rocker centering device at the rear ends of the truck frame. delta trucks were soon enlarged to carry four trailing wheels, and later six.