The British Rail Class 56 is a type of diesel locomotive designed for heavy freight work. It is a Type 5 locomotive, with a Ruston-Paxman power unit developing 3,250 bhp (2,423 kW), and has a Co-Co wheel arrangement. Enthusiasts nicknamed them "Gridirons" (or "Grids" for short), due to the grid-like horn cover on the locomotive's cab ends fitted to nos. 56056 onwards.

The Class 56 fleet was introduced between 1976 and 1984, a total of 135 examples were manufactured. The first 30 locomotives (56001 - 56030) were built by Electroputere in Romania, but these typically suffered from poor construction standards and many were withdrawn from service early for extensive rebuilding before re-entering revenue service. The remaining 105 locomotives were built by British Rail Engineering Limited (BREL) at Doncaster Works (56031 to 56115) and Crewe Works (56116 to 56135).

In April 2019, GBRf announced that several Class 56 locomotives would be rebuilt as Class 69s by Progress Rail at its Longport facility. The rebuild scheme involves the replacement of the original Ruston-Paxman RK3 engine with the EMD 710 powerplant, while newer electronic control systems based on those present on the Class 66 were also installed. While the external appearance and many elements were retained, much of the internal systems were replaced with those sourced from the Class 66.