309 Class 37 locomotives were built for British Rail starting with D6700 in December 1960 through to D6608 in November 1965. They were all fitted with an English Electric V12 power unit rated at 1750 hp supplying an English Electric DC main generator.
In 1974 the BR TOPS (Total Operation Processing System) came into being and all locomotives were allocated a 5 figure number (the first 2 being the Class number).
During the 1985 a large part of the class 37 fleet underwent refurbishment into the numerous sub classes, with various modifications to suit the now very varied work they were carrying out.
In 1986 BR were seeking to dramatically reduce the locomotive types in use and looked to creating a Class 38 locomotive. In order to do this BR decided to fit various power units and alternators into existing locomotives in order to gain experience before placing a large order. And thus the Class 37/9’s were created by fitting locos:
37901 – 904 with Mirrlees six cylinder 1800 hp power units and Brush electronic equipment
37905 & 906 with six cylinder GEC / Ruston 1800 hp power units and GEC electronic equipment.
The class was allocated mainly on South Wales Steel traffic, although they did eventually stray far and wide. The Class 38 development went no further, however, and BR decided to build the 100 strong Class 60 fleet (fitted with a bigger version of the Mirrlees power unit).
The Class 37/9’s later became known as Slugs due to their slow application of traction power.
37901 "Mirrlees Pioneer”
37901 was built at the Vulcan Foundry in Newton-Le-Willows (Merseyside) in July 1963 as D6850 and was initially allocated to Cardiff Canton shed, then to Haymarket (Edinburgh) and then Eastfield (Glasgow). In April 1974 it was renumbered as 37150.
Refurbished and re-engined at Crewe it emerged 30th October 1986 as 37901 and was named "Mirrlees Pioneer” at Cardiff on the 3rd December that year. 37901 worked until 1998 when it went into storage until being sold for preservation in 2003, initially at the Llangollen Railway and later at the East Lancashire Railway.
Out of the six Class 37/9 locomotives the only other survivors are the two Ruston fitted locos, 37905 which is also based at the railway, and 37906 which is owned by Europhoenix Freight.