The leading wheels on a 4-4-0 'American Standard' replica.
A leading wheel (or leading axle) is a set of two or more wheels located in front of the driving wheels on a locomotive. The axles of the leading wheels are normally located in a truck. Leading wheels are used to help the locomotive negotiate curves and to support the smokebox on steam locomotives.
Importantly, the leading truck does not only have rotational turning capacity, but also slight side to side motion as well. It must be free to slip sideways to a small extent to follow curves accurately.
The first locomotive to use a swiveling pilot truck was the Camden & Amboy's John Bull. A two wheel leading truck was fitted to the engine to help guide it around curves, and it was successful enough to be modified and copied on countless steam locomotives.