The design specification was based upon the Class 47 bodyshell, with the omission of redundant features such as the headcode panel. Power came from a Ruston-Paxman unit that was similar to that fitted in the Class 50 locomotives, which provided the capability for heavy haul freight. Air brakes were fitted for the first time in a British Rail engine and the electrics were improved in a number of areas, resulting in a more robust power unit. Nicknamed as Gridirons or "Grids", the Class 56 displaced the Class 20 and Class 47 locomotives and commonly undertook the transportation of iron ore, coal, aggregate and steel across the Eastern and London Midland Regions and on the Western Region for aggregate duties.

Strong and reliable performers, the locomotives were popular, but needed heavy maintenance, particularly when compared against the Class 66 locomotives that came to displace them in service. The introduction of the Class 60 locomotives saw a gradual reduction in the use of the Class 56 locomotives and with the arrival of the Class 66 locomotives, with their greater availability and lower maintenance, the Class 56 fleet were placed into storage between 1998 and 2004.

Most of the locomotives remain in depots around the UK, despite being variously sold for scrap and then re-acquired and reinstated in service. Colas Railfreight operate a small fleet on a rotating basis and British American Railway Services and UK Rail Leasing have a number available for working. Five locomotives, 56006, 56097, 56098, 56301 and 56302 have been preserved. No. 56047 was built in July 1978 by BREL in Doncaster. The locomotive served with BR and later Transrail until being put into storage in May 1999. The locomotive was scrapped in December 2003.