Historical:

The concept of the Mechanical Horse tractor unit with an easily detached articulated trailer was conceived in the early 1930s by Napier & Son. The London and North Eastern Railway had approached Napier's, the quality car and aero-engine makers for an answer to the problem of replacing horses for local haulage purposes, while retaining the flexibility of changing the wagons and the maneuverability of the horse and wagon.

Info:  Scammell Mechanical Horse – Royal Army Service Corps (RASC) Flatbed Trailer, World War II