A Bissel truck (also: Bissell truck or Bissel bogie) is a single-axled bogie which pivots towards the centre of a steam locomotive to enable it to negotiate curves more easily. Invented in 1857 by Levi Bissell and usually known as a pony truck, it is a very simple and common means of designing a carrying axle.
Bissel truck Wikipedia
A locomotive with a Bissell axle is able to both turn about its vertical axis and swing radially to the side, movements advantageous to steam locomotives because their position on the track is dictated by the driving or coupled wheels.
The Bissel truck also helps stabilize a train in a turn, where centrifugal force causes a locomotive to lean away from the track. It features a pair of inclined planes which mate with an opposing set on the engine's frame where the two join. The more a truck moves to the side, the greater the lift to the outside of the locomotive, canting it slightly into the curve. Though the system was effective, casting and machining its sloping surfaces was expensive.
A refinement, the Hudson-Bissell truck, delivered the same result using less costly components. Instead of resting upon opposing planes, the engine frame is joined to the truck by two swing links. As the truck pivots sideways the outside of the locomotive is elevated, a practical modification used right to the end of steam.
The pony truck can move radially around a real or virtual pivot. When the pivot is situated at a point inside the truck, the truck is called a bogie. What makes it a Bissel bogie is the pivot being placed outside to the rear or fore.
In Mallet locomotives, the forward engine employed a special, very large, Bissell truck.
Examples of steam engines fitted with Bissell trucks include the German DRG Class 64 and Class 99.73-76 locomotives.
Even some older electric locomotives have Bissell trucks, if the driving axles are located in the main frame rather than the now usual bogies.
A British example was the London and North Western Railway 0-4-2 tank locomotive which was known as a "Bissell tank" or "Bissell truck tank". It was also used on the South African Class 4E electric and Class 32-000 and 32-200 diesel-electric locomotives.