British Rail Class 121

Sixteen Class 121 single-car driving motor vehicles were built from 1960, numbered 55020–55035. These were supplemented by ten trailer vehicles, numbered 56280–56289 (later renumbered 54280–54289). They had 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 now endorsed by current operator Chiltern Railways).

British Railways service
The Class 121 were built for use on the Western Region of British Rail. They were used on various lightly used branch lines, including the Looe branch line (and various other branch lines) in Cornwall, the branch lines off the main line in the Thames Valley, the Severn Beach line in Bristol and the Greenford Branch Line in West London.

Class 121s were built with distinctive exhaust pipes, designed to fit around a large four-digit route headcode indicator box at roof level. Use of the headcode indicator box was discontinued in the 1970s, so the indicator was removed and the hole plated-up when the units were refurbished.

They also had a small destination indicator inside the top of the centre cab window.

The similar Class 122 units had the small destination indicator in the cab roof in place of the large headcode box. They were used on the other regions of British Rail.

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. There are three examples left in revenue-making service.

Chiltern Railways
In 2003, Chiltern Railways reintroduced "Heritage" diesel multiple units on its Aylesbury to Princes Risborough shuttle service. For this purpose, unit 121020 (vehicle 55020) 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 Class 165 "Turbo" unit for refurbishment. This unit has been fitted with secondary (magnetic) door locking and other safety features, and thus was exempt from the 30 November 2005 deadline for the withdrawal of all Mark 1 vehicles.

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 a green livery and is used on Aylesbury to Princes Risborough services when required.

In March 2013, Chiltern Railways acquired 121032 from Arriva Trains Wales for use as a spare parts donor.

Network Rail
Network Rail operates a small fleet of former Class 121 vehicles for various departmental roles. These have been reclassified as Class 960 departmental units. Duties include route learning, video surveying of track and sandite application. Two members of the fleet were also stabled at Severn Tunnel Junction for emergency evacuation of the Severn Tunnel, where they were never used, and eventually disposed of. Units are painted in various liveries including Network SouthEast (NSE), South West Trains blue/red, Railtrack's brown livery, Railtrack's blue/green livery, and Network Rail's new all-over yellow livery. In addition, two units have been specially repainted in old BR liveries, namely maroon and blue/grey. The units stabled at Aylesbury are jointly used by Chiltern Railways for route learning.

Modern day operations gallery
The current fleet is listed below:

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 Gospel Oak to Barking. 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) 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 to Sudbury. These returned to Bletchley in 1997/98. Two of the units (121027/029) were repainted into Silverlink's purple and green livery, and the third (121031) 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

The units were replaced on Silverlink duties in 2001 by Class 150 "Sprinter" units cascaded from Central Trains. The units have all since been transferred to departmental duties.

Arriva Trains Wales
In 2006, unit 121032 was purchased by Arriva Trains Wales for use on the Cardiff Bay Shuttle. The unit has been repainted into Arriva colours and was hauled to Wales in June 2006 to enter service in July. The unit finally entered service on 16 August 2006 but was withdrawn 3 days later due to a major problem with the engine. The unit re-entered service on the Cardiff Bay line on 14 September 2006. The unit has been regularly seen on the Barry Tourist Railway during training purposes for the unit. In March 2013, 121032 was sold to Chiltern Railways for use as a spare parts donor.

Preservation
"Bubble Cars" have proved popular for preservation on heritage railways.

Fleet details
Remarkably, considering the age of the fleet, the oldest unit (55020) is still in daily service, whilst the youngest vehicle (55035) was the first to be dismantled. As of early April 2011, 55021 (960021) and 55030 (960013) have also been dismantled. This could change in the future, due to the former Severn Tunnel Emergency Train sets are being stripped for spares at Cardiff to keep 55032 running.