Historically, all British Rail (BR) locomotives were allocated to a particular depot. The 1955 Modernisation Plan paved the way for the changeover from steam to diesel locomotives on Britain’s railways. Initially, the modern diesel locomotives were allocated to the same depots as those used by steam locomotives and forced to share the same facilities. This often caused problems since these environments were by their very nature exceedingly dirty. This often prejudiced the reliability of the new diesel locomotives, which required far higher levels of cleanliness.
Gradually in the early 1960s more modern, custom-built facilities, such as Finsbury Park and Laira traction maintenance depots (TMDs) were constructed. These were far more suited to the maintenance of modern diesel locomotives with improved levels of cleanliness and a workforce specifically trained in their maintenance.