The British Rail Class 455 is a Mark 3-based electric multiple unit (EMU) passenger train built by BREL. Upon their introduction, they replaced the Class 508s and also the ageing Southern Railway slam-door stock on commuter services out of London Waterloo. They were mostly used in Greater London, Surrey and also by Southern

The Class 455 was originally to be classified as the Class 510, basically a 750 V DC version of the Class 317. However, as the chopper control system at the time was not considered robust enough for the electrically rougher third-rail Southern Region, they were fitted with GEC Traction camshaft-control systems instead, and the Class 510 designation was changed to Class 455.

The class has the same bodyshell as the Class 317 and Class 318, which is based on the MKIII, but with a steel construction as opposed to aluminium alloys like PEP-based classes. As they were designed for inner suburban services, they do not feature first-class seating, air conditioning or toilet facilities, and are restricted to 75 mph. 

They are currently being replaced by the Class 701 EMU on the South Western Railway network.

One full unit has been preserved. A significant number have been scrapped.