Initially numbered D400–D449 and known as English Electric Type 4s, the locomotives were purchased outright by British Rail (BR) at the end of the lease and became Class 50 in the TOPS renumbering of 1973.

The origins of the Class 50 lie in an invitation from the British Transport Commission (BTC) to manufacturers to produce a design for a lightweight Type 4 diesel locomotive with a gross power output of at least 2,500 hp.

[3] However, before the prototype could be completed, English Electric's design was rejected by the BTC, primarily on the grounds that the 16CSVT was unproven, and doubted its ability to produce the required power levels.

The DP2 design earned a reprieve in 1965, when the National Traction Plan of that year identified a need for an additional 50 Type 4 locomotives.

[5] English Electric intended to build the new batch as similar to DP2 as possible[6] but the British Railways Board (successor to the BTC) had produced a standard locomotive cab with a flat front and headcode box and also had specific requirements relating to the engine room and other equipment.

[7] Unusually, the ownership of the locomotives remained with the manufacturer and they were operated by British Rail on a 10-year lease which included certain stipulations relating to availability.

Once the electric service was introduced as far as Preston in 1973, this double-heading by Class 50s transferred there, although poor availability often resulted in single-heading with consequent delays.

From 1977, British Rail introduced InterCity 125s onto the Great Western Main Line which began the displacement of the Class 50 fleet onto other routes, such as services to Birmingham New Street from London Paddington and Bristol Temple Meads.

However, due in part to the over-complexity of the design, the class was plagued with reliability problems which resulted in frequent service failures and cancellations.

The work involved simplifying the complex electronics and removing redundant features such as slow speed control and rheostatic braking.

The filtration system was fundamentally sound and widely used in other countries; the problems arose because relative humidity had not been taken into account at the design stage.

The class were again used for Western Region services on the GWML out of Paddington, and on the West of England Main Line from Waterloo to Salisbury and Exeter.

[9] In 1984, 50007 Hercules was repainted into lined Brunswick green livery and renamed Sir Edward Elgar, to commemorate the 150th anniversary of the Great Western Railway (GWR).

In 1986 the West of England Main Line came under the control of the Network SouthEast (NSE) sector, which saw the introduction of their bright blue, red and white livery.

Following an intermediate overhaul at Doncaster, the loco was released painted in grey primer, and worked a service train back to the Western Region as far as Bristol on 15 July 1986, prior to running to Old Oak Common depot for the NSE livery to be applied.

To this end, 50049 Defiance was renumbered to 50149, equipped with modified Class 37, lower-geared bogies and outshopped in the new trainload grey livery with Railfreight General decals.

Ironically, the electronic anti-wheelslip equipment, with which the entire class had originally been built and which would have been key to the success of this experiment, had been removed during the refurbishment process.

The Portuguese Railways, CP, bought ten locomotives similar to the BR Class 50, but built to Iberian track gauge (1,668 mm).

While still an integral part of the National Railway Museum, it is set to return to service pulling tourist trains on the Douro River Line.

The intention was to restore them by using industrial sponsorship money to build an engineering centre and overhaul the components, making brand new ones where necessary to overcome lack of availability of some parts unique to the original design.

50040 could have been suitable for cosmetic restoration, but after many years untouched and in a derelict state at the Coventry Railway Centre, it finally had all remaining parts stripped for spares and was transported to Sims Metals of Halesowen for scrapping.

Although cosmetically very smart, the loco was by this stage unrestorable and although an ambitious private individual did try, this effort soon came to naught and it was scrapped to provide parts for 50026 Indomitable.

50031 was also the first to operate on the mainline, hauling the Past Time Railtours Pilgrim Hoover train from Birmingham International to Plymouth on 1 November 1997.

However, with changes in the UK's Rail Access regulations (requiring fitment of additional equipment: TPWS/OTMR/GSMR) some of these locos are no longer of a standard to continue mainline operation.

One locomotive, 50017, was hired to Venice Simplon Orient Express (VSOE) to work the Northern Belle service from Bath to Manchester Victoria.

[clarification needed] In 2003, the National Railway Museum decided to dispose of 50033 due to an inability to commit to maintenance and storage costs.

However, soon after it was sent on to the Tyseley Locomotive Works as part of a proposed move to the RailSchool project and Royal Docks Heritage Railway in North Woolwich, London.

This situation ended in February 2018, when an agreement between the Birmingham Railway Museum and the Fifty Fund was reached to return the locomotive to operational use.

October 2018 saw the Severn Valley Railway host the 'Class 50 Golden Jubilee' gala, celebrating fifty years since the class was introduced into traffic.

Accurascale announced in November 2022 that they would be producing a newly tooled OO gauge Class 50 model, based on a 3D laser scan taken of locomotive 50017 at the Great Central Railway in 2019.