Wiktionary
trailing axle
n. On a steam locomotive, an axle connecting a coupled pair of unpowered wheels located behind the driving wheels.
Usage examples of "trailing axle".
If an 'eight coupled' loco is too heavy for the relatively flimsy tracks we have in service, we could construct a 'Decapod' 2-10-0 (the driving weight is spread over five axles instead of four) or a 'Santa Fe' 2-10-2 (the trailing axle permits a larger firebox, hence more steaming capacity).