The pin or spindle on which a wheel revolves, or which revolves with a wheel.

A transverse bar or shaft connecting the opposite wheels of a car or carriage; an axletree.

A spindle on which a wheel revolves. The axles house the front and rear braking system. (In Figure 1, the axle is indicated in purple.)  Figure 1. Axle (Front)

The steel shaft on which the locomotive wheels are mounted. The axle not only holds the wheels to gauge, but also transmits the load from the Axle-boxes to the wheels.