The British Rail Class 121 is a single-car double-ended diesel multiple unit. 16 driving motor vehicles were built from 1960, numbered 55020–55035. These were supplemented by ten single-ended trailer vehicles, numbered 56280–56289 (later renumbered 54280–54289). They have a top speed of 70 mph, with slam-doors, and vacuum brakes. The driving motor vehicles were nicknamed "Bubble Cars" by some enthusiasts (a nickname endorsed and made official by final passenger service operator Chiltern Railways). The Class 121 is Britain's longest serving DMU, operating in passenger service for 57 years until 2017. British Railways service The Class 121 vehicles were introduced in 1960 for use on the Western Region of British Rail. They were used on various lightly used branch lines in Cornwall including the Looe branch, the branch lines off the main line in the Thames Valley including the Greenford Branch Line, the Bridport branch line (closed 1975), and the Severn Beach line in Bristol. In 1978 all of the units were still allocated to Western Region depots. Unlike the earlier (but similar) , which had a small destination indicator in the roof dome, Class 121 had a four-character headcode box in the roof dome, with the destination indicator inside the top of the centre cab window. Current operations Upon privatisation of Britain's railways, the Class 121 fleet was only operated by one passenger company, namely Silverlink, with several more units in departmental duties with Railtrack. Since Chiltern Railways retired its final two units in May 2017, there have been no examples left in revenue-making service. However, Network Rail operates one Class 121 for various departmental roles. This has been reclassified as a Class 960 departmental unit. Duties include route learning, video surveying of track and sandite application. In addition, the unit has been specially repainted in old BR blue livery. The unit is based at Aylesbury and was jointly used by Chiltern Railways for route learning. Past operations Silverlink Silverlink inherited a small fleet of four "Bubble Cars". They were mainly used on the Marston Vale Line from Bletchley to Bedford, as well as non-electrified lines in North London, such as the Gospel Oak to Barking line. The units replaced the previous fleet of Class 108 and Class 115 units, and were supplemented with a fleet of Class 117 units cascaded from Thames Valley services. The four units, nos. 55023/27/29/31, were based at Bletchley depot, where staff repainted set L123 (55023)