British Rail Class 121

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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(s) Chiltern Railways
Network Rail
Line(s) served Princes Risborough to Aylesbury Line
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)
  • DMBS: 37 long tons 8 cwt (83,800 lb or 38 t)
  • DTS: 29 long tons 7 cwt (65,700 lb or 29.8 t)
Coupling system Screw[1]
Multiple working Blue Square
Track gauge 4 ft 8 12 in (1,435 mm)

Sixteen Class 121 single-car, double-ended 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 now endorsed by current operator Chiltern Railways).[2]

The Class 121 is Britain's longest serving DMU, having been in service for 55 years as of 2015.

British Railways service[edit]

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, the Bridport branch line (closed 1975), 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[edit]

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[edit]

In 2003, Chiltern Railways reintroduced "Heritage" diesel multiple units on its Aylesbury to Princes Risborough shuttle service. For this purpose, unit 121 020 (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 "Network Turbo" unit. 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. 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 121 034, previously based at Tyseley Loco Works, was (less extensively) refurbished for use by the Birmingham Railway Museum. It is painted in BR Green livery and is also used on Aylesbury to Princes Risborough services, when required.[3]

Both units are 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 121 032 from Arriva Trains Wales, which was sold on in 2015, for preservation.

Network Rail[edit]

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 jointly used by Chiltern Railways for route learning.

Current fleet[edit]

The current fleet is listed below:

Unit no. Vehicle no. Livery Use Status
Current no. Previous no.
121 020 55020 977722 Chiltern Railways blue - In traffic (based at Chiltern Railways Aylesbury depot) following departmental use as 960 002
960 014 977873 55022 BR Blue/Grey Route-learning unit In traffic (based at Chiltern Railways Aylesbury depot)
121 034 55034 - British Railways coaching stock green - In traffic (based at Chiltern Railways Aylesbury depot), from preservation at Tyseley Locomotive Works, as 121 034

Past operations[edit]


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 (121 027/029) were repainted into Silverlink's purple and green livery, and the third (121 031) 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.

Arriva Trains Wales[edit]

In 2006, unit 121 032 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. In March 2013, following further extensive periods out of traffic, 121 032 was finally withdrawn from service, due to a terminal engine fault. It was subsequently transferred to sister company Chiltern Railways and moved to Aylesbury at the beginning of 2014 and was later sold into preservation at the Wensleydale Railway, moving north at the beginning of May 2015.


"Bubble Cars" have proved popular for preservation on heritage railways. Including some in the class still in Service.

Vehicle no. Unit no. Vehicle type Location Comments
Original no. Departmental no.
56287 - L211 DTS Colne Valley Railway -
56289 - L263 DTS East Lancashire Railway -
55023 - L123 DMBS Chinnor & Princes Risborough Railway Repainted into BR Green livery whilst based at Bletchley depot.
55024 977858 L124 DMBS Chinnor & Princes Risborough Railway Repainted into BR Coaching Stock maroon livery while based at Aylesbury depot. Awaiting restoration.
55027 977975 P127 DMBS Ecclesbourne Valley Railway Previously Severn Tunnel Emergency train 960 302, sold to Barry Tourist Railway around January 2014 and then on to the Ecclesbourne Valley Railway in June 2014.
55028 977860 L128 DMBS Swanage Railway Recently sold by South West Trains, it was used as a departmental route-learning unit. Currently in B.R "Heritage" green with "D.M.U Whiskers" livery after a major overhaul was undertaken after its arrival. Currently at Eastleigh under overhaul to allow eventual use on the mainline connection to Wareham.
55029 977968 L129 DMBS Rushden, Higham and Wellingborough Railway Recently sold by Colas Rail from store at Rugby. It was converted there into non-passenger carrying stock. Damaged in fire, May 2015.[4]
55031 977976 T002 DMBS Ecclesbourne Valley Railway Previously Severn Tunnel Emergency train 960 303, sold to Barry Tourist Railway around January 2014 and then on to the Ecclesbourne Valley Railway in June 2014.
55032 T003 DMBS Wensleydale Railway In Arriva Trains Wales livery, stored
55033 977826 T004 DMBS Colne Valley Railway Painted in experimental MidLine livery. It's currently in B.R "Heritage" green with "D.M.U Whiskers" livery.
55034 977828 T005 DMBS Chiltern Railways Initially preserved at Birmingham Railway Museum. Returned to capital stock with Chiltern Railways as 121 034.

Fleet details[edit]

Table of orders and numbers
Lot No. Type Diagram Qty Fleet numbers Notes
30518 Driving Motor Brake Second (DMBS) 512 16 55020–55035
30519 Driving Trailer Second (DTS) 513 10 56280–56289
Present fleet
Class Operator No. in Service Year Built Cars per Set Unit nos. Notes
Class 121 Chiltern Railways 2 1960 1 121 020, 121 034
Network Rail 1 121 022 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 at July 2014 55021 (960 021) and 55030 (960 013) have also been scrapped.


  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. 
  4. ^

External links[edit]