The Class 37 locomotives had a low axle load and a route availability of five (RA5), so could travel virtually anywhere on the national rail network.
Several subclasses were developed, to enable a longer their lifespan.
Ninety locomotives still exist, some in active mainline service, some in long-term storage, and some in preservation.
In October 1968, the ‘D' prefix was dropped, and during October 1971, 6915 moved to Tinsley depot (41A) near Sheffield in South Yorkshire.
Two years later, in September 1973, it moved to Stratford depot (SF) in East London, and spend the next eight-and-a-bit year working on the Great Eastern lines from London into East Anglia.
This included servicing Essex and Thameside's many freight facilities in the Essex and north Thameside area, including travelling around the north London line into the Southern and Western regions.
During this time, in March 1974, the loco was renumbered 37215.
In January 1982, it returned to Tinsley depot (TI), and spent the next six years hauling trains around Yorkshire, including regular Saturday seaside trips from the Sheffield area to resorts such as Blackpool, Scarborough and Skegness.
Between November 1987 and May 1988, the loco moved quickly from Tinsley (TI) to Immingham (IM) then Stratford (SF) and finally
Cardiff Canton (CF), where it became part of the Trainload Petroleum fleet, spending the next four years working heavy oil trains out of South Wales to terminals across the south of England.
It usually worked in a pair with another Class 37.
37215 went into storage in August 1992, moving to Inverness depot (IS) and being officially withdrawn on 16 July 1993.
It was subsequently bought for preservation by The Growler Group then bought it for preservation, and moved it to GWR's Toddington site road.
It arrived on 23 June 1994, where work began to restore it to operational condition.
The locomotive returned to full working order in July 1998 and after several test runs, its paintwork was finished on 25 August 1998.
In 2007, 37215 was repainted in British Rail blue livery as the class wore during the 1970s and '80s.
Tickets for GWSR's summer diesel gala are available online on
the Railway's website.
The event will feature up to eight diesel locomotives and a diesel multiple unit (DMU) in operation.
The locomotive and carriage & wagon workshops will be open for behind-the-scenes visits.
Other activities will take place at the railway's period stations.