British Rail Class 121
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. They have a top speed of, with slam-doors, and vacuum brakes. The driving motor vehicles were nicknamed "Bubble Cars" by some enthusiasts.
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 branch, the branch lines off the main line in the Thames Valley including the Greenford branch line, the Bridport branch line, and the Severn Beach line in Bristol. In 1978 all of the units were still allocated to Western Region depots.Unlike the earlier , 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.As of 2026, two units are owned by Locomotive Services Limited:
- 55022 was acquired in 2016. It has been heavily refurbished and, as of September 2023, operates as part of the chairman's private fleet, bearing the name 'Flora'.
- 55034 was acquired for use as a route learner in 2017. It has since been loaned to heritage railways; as of June 2024, it is on loan to the Cholsey and Wallingford Railway, having previously been loaned to the Ecclesbourne Valley Railway.
Past operations
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 and units, and were supplemented with a fleet of units cascaded from Thames Valley services.The four units, nos. 55023/27/29/31, were based at Bletchley depot, where staff repainted set L123 into its original British Railways green livery. The others remained in obsolete Network SouthEast livery. In 1996, set L123 was withdrawn from traffic, and the other three were hired to Great Eastern for use on the Sudbury branch. These returned to Bletchley in 1997/98. Two of the units were repainted into Silverlink's purple and green livery, and the third was repainted into Network SouthEast livery. The three units also received names from withdrawn Class 117 units.
- 121027 - Bletchley TMD
- 121029 - Marston Vale
- 121031 - Leslie Crabbe
Chiltern Railways
In 2003, Chiltern Railways reintroduced "Heritage" diesel multiple units on its Aylesbury to Princes Risborough shuttle service. For this purpose, unit 121020 was purchased from Network Rail, and heavily refurbished to allow it to operate passenger services. It was repainted into Chiltern Railways blue livery. The introduction of this unit allowed the release of a "Network Turbo" unit. This unit has been fitted with secondary door locking and other safety features, and thus was exempt from 30 November 2005 deadline for the withdrawal of all Mark 1 vehicles. Electronic destination indicators and internal passenger information systems were recovered from "Network Turbo" 165 032 at refurbishment and fitted to this unit. The external exhaust pipes were rerouted through the brake van area.In May 2011, a second "Bubble Car" was reintroduced to regular service. Unit 121034, previously based at Tyseley Loco Works, was refurbished for use by the Birmingham Railway Museum. It is painted in BR Green livery and was also used on Aylesbury to Princes Risborough services, when required.
Both units were additionally used on shuttles, as required, between Aylesbury and Quainton Road stations, on the occasion of events at the Buckinghamshire Railway Centre.
In March 2013, Chiltern Railways acquired 121032 from Arriva Trains Wales. It sold the unit for preservation in 2015.
Chiltern's two final units were withdrawn from service on 19 May 2017, due to difficulties in obtaining spare parts given the age of the units, which were more than 50 years old. They were subsequently sold.
Preservation
Class 121s have proved popular for preservation on heritage railways.Order details
| Lot No. | Type | Diagram | Qty | Fleet numbers | Notes |
| 30518 | Driving Motor Brake Second | 512 | 16 | 55020–55035 | |
| 30519 | Driving Trailer Second | 513 | 10 | 56280–56289 |