 He had become convinced that the conventional pattern of locomotive was seriously deficient; they wasted weight on unpowered wheels (the maximum tractive effort a locomotive can exert is a function of the weight on its driving wheels) and on a tender that did nothing but carry fuel and water without contributing to the locomotive's adhesive weight.Furthermore, the standard locomotive had a front and back, and was not intended for prolonged driving in reverse, thus requiring a turntable or wye at every terminus.Fairlie's answer was a double-ended steam locomotive, carrying all its fuel and water aboard the locomotive and with every axle driven.