121020 at Bewdley on the Severn Valley Railway in October 2004. © Phil Scott.
Built between 1960 and 1961, a total of 16 Class 121 single car DMUs were built by Pressed Steel for use on the Western Region of British Rail. Featuring slam-doors, vacuum brakes and a top speed of 70mph; the 'Bubble Cars' were used on lightly used branch lines and thanks to their affordability, helped to keep rural services running. Remarkably, they have the honour of being Britain's longest serving DMU, having only been fully withdrawn in 2017. These versatile units have proven to be popular on heritage lines too, with 12 currently preserved.
(Information Provided by Wikipedia)