Trailing wheel

On some large locomotives, a booster engine was mounted on the trailing truck to provide extra tractive effort when starting a heavy train and at low speeds on gradients.

As demand for more powerful locomotives increased, trailing wheels began to be used to support the crew cab and rear firebox area.

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.

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.