They were originally numbered D1-D10 and named after mountains in England and Wales, and, along with the similar Class 45 and 46 locomotives, they became known as Peaks.

They were originally designed to have a Co-Co wheel arrangement, but it proved impossible to keep below the 20 long tons (20,000 kg) axle loading limit imposed by the British Railways Civil Engineer.

The main problem with the exhausters was that they were mounted in the centre of the engine room and so had to be lifted through the roof by crane, taking 8–12 hours to change.

[8] Once the class 45 units were available, the locomotives were re-assigned to Toton and freight duties, and the steam heating boilers were removed.

The class worked regularly over the West Coast Main Line for a couple of years prior to its electrification, and also between London St Pancras and Manchester Central.