British Rail Class 121

British Rail Class 121
British Rail Class 121

Class 121 in Network South East livery at Paddington station – note the exhaust pipes and plated-over headcode box (1988)
In service 1960–present
Manufacturer Pressed Steel Company
Number built 16 motor cars,
10 trailer cars
Number preserved 5 motor, 2 trailer
Formation Single car
Fleet numbers 55020–55035
Operator Arriva Trains Wales,
Chiltern Railways,
Network Rail
Car body construction Steel
Car length 64 ft 6 in (19,660 mm)
Width 9 ft 3 in (2,820 mm)
Height 12 ft 4 in (3,760 mm)
Maximum speed 70 mph (110 km/h)
Weight DMBS: 37 tons 8 cwt (83,800 lb or 38.0 t),
DTS: 29 tons 7 cwt (65,700 lb or 29.8 t)
Coupling system Screw[1]
Multiple working Blue square
Gauge 4 ft 8 12 in (1,435 mm)

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).[2]


British Railways service

Preserved W55033 at the Colne Valley Railway in original green livery, with yellow "speed whiskers" (warning stripes) on the cab front. This unit differs from the rest of the class by the fitment of a small route indicator box above the centre window, like a Class 122, rather than a large headcode box.

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, 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 two examples left in revenue-making service.

Arriva Trains Wales

In 2006, unit 121032 was purchased by Arriva 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 Island Railway during training purposes for the unit.

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.[3]

Network Rail

Network Rail (NR) 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 in anger 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:

Unit no. Vehicle no. Livery Use Status
Departmental no. Former no.
960010 977858 55024 BR Maroon Sandite unit In traffic (At Aylesbury)
960011 977859 55025 Railtrack Blue/Green (Ex Video survey unit, now Route Learner) Stored
960013 977866 55030 NSE Sandite unit Scrapped in 2011 (whilst at Aylesbury)
960014 977873 55022 BR Blue/Grey Route-learning unit In traffic (At Aylesbury)
960021 977723 55021 Railtrack Brown Sandite/Route-learning unit Scrapped in 2011 (whilst at Aylesbury)
960302 977975 55027 NR Yellow Severn Tunnel emergency unit Stored
960303 977976 55031 NR Yellow Severn Tunnel emergency unit Stored

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 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.

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.


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

Vehicle no. Unit no. Vehicle type Location Comments
Original no. Departmental no.
54287 - L211 DTC Colne Valley Railway -
54289 - L263 DTS East Lancashire Railway -
55023 - L123 DMBS Chinnor & Princes Risborough Railway Repainted into BR Green livery whilst based at Bletchley depot.
55026 977824 P126 DMBS Swansea Vale Railway -
55028 977860 L128 DMBS Swanage Railway Recently sold by South West Trains
55029 977968 L129 DMBS Rushden, Higham and Wellingborough Railway Recently sold by Colas Rail from store at Rugby.
55033 977826 T004 DMBS Colne Valley Railway Painted in experimental MidLine livery.
55034 977828 T005 DMBS Tyseley Locomotive Works -

Fleet details

Class Operator No. in Service Year Built Cars per Set Unit nos. Notes
Class 121 Arriva Trains Wales 1 1960 1 121032
Chiltern Railways 2 121020, 121034
Network Rail 8 121021/022, 121024/025,
121027/028, 121030/031
Reclassified as Class 960

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.


  1. ^ "Mechanical And Electrical Coupling Index". Rail Safety and Standards Board. Retrieved 2010-12-20. 
  2. ^ [1] Chiltern Railways website. Retrieved May 2010
  3. ^ Miller, Mark (18 May 2011). "Chiltern's latest trains are 50 years apart". RAIL (670): 11. 

External links

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