The Class 50 locomotives, built by English Electric at the Vulcan Foundry in Newton-le-Willows between 1967-1968 were the last purpose-designed mixed-traffic locomotive built in the UK. 50 examples were built in total. Upon their introduction they were used on the West Coast Main Line (WCML). The aim was to speed-up passenger timings on the then non-electrified portion up to Glasgow, and to haul express passenger trains at 100 mph. By 1974, the entire line had been electrified so the locomotives were re-deployed to other regions as had been planned. When delivered, the Class 50s were leased to British Rail by a subsidiary of English Electric. They were later sold to the British Railways Board.

The Class 50s proved to be incredibly powerful, indeed following the full electrification of the WCML and the introduction of more powerful DMUs in the late 1980´s they were almost pointlessly powerful, and they began to be withdrawn as their roles were handed to more efficient DMUs. All examples of the Class were named which was rather unusual for corporate BR diesel locomotives, and in the end 18 examples have been preserved.

No. 50049 started its working life as D449 in the Stoke Division depot (D05) after being constructed at the Vulcan Foundry by English Electric in 1968. The locomotive was the last of the Class 50s to be built and the last diesel locomotive until the Class 56s were constructed in 1977. D449 was renumbered to 50049 under TOPS in 1974. After a reallocation to Bristol Bath Road and then to Laira in 1977, the locomotive was named 'Defiance' at Laira in 1978. No. 50049 was outshopped as 50149 in a Railfreight grey livery in 1987 for heavy stone and china clay haulage. The bogies were also modified for this purpose. No. 50149 became 50049 once more shortly before withdrawal from service in 1991. Defiance was saved for preservation and moved to the Severn Valley Railway in 1999. Now mainline registered and in a GBRf livery, Defiance carries out rolling stock moves and other duties. As of June 2024, Defiance carried out its last duty in the GBRf livery and will be repainted at Eastleigh.