British Rail Class 121

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British Rail Class 121 Bubble Car
Chiltern Railways 121020 at Quainton Road in May 2010
121032 Interior.JPG
Interior of 121032
In service1960–2017
ManufacturerPressed Steel Company[1]
Order no.
  • 30518 (DMBS)
  • 30519 (DTS)[2]
Family nameFirst generation
ReplacedSteam locomotives and carriages
GWR railcars
Entered service1960
Number built
  • 16 (DMBS)
  • 10 (DTS)
Number preserved
  • 12 (DMBS)
  • 2 (DTS)
Number scrapped
  • 4 (DMBS)
  • 8 (DTS)
  • Single car (with trailer)
  • DMBS(+DTS)
  • DQ221 (DMBS)
  • DS201 (DTS)[3][4]
  • DX201 (DMBS, by 1994)[5]
Fleet numbers
  • 55020–55035 (DMBS)
  • 56280-56289 (DTS, as built)
  • 54280-54289 (DTS, renumbered)[1]
  • 65 (DMBS)
  • 91 (DTS)[4]
Operator(s)British Railways
Network SouthEast
Regional Railways
Network Rail
Arriva Trains Wales
Chiltern Railways
Line(s) servedPrinces Risborough to Aylesbury Line
Car body constructionSteel[1]
Car length64 ft 6 in (19,660 mm) (over body)[4]
Width9 ft 0 in (2,740 mm) (over body)[4]
Height12 ft 4 12 in (3,772 mm)[4]
Articulated sectionsSingle car (2)
  • 46 ft 6 in (14.17 m) (bogie centres)
  • 8 ft 6 in (2.59 m) (bogies)[4]
Maximum speed70 mph (110 km/h)[4]
  • 38 t (37 long tons; 42 short tons) (DMBS)
  • 30 t (30 long tons; 33 short tons) (DTS)[1]
Prime mover(s)
  • 2 × AEC of 150 hp (110 kW) per DMBS
  • or
  • 2 × Leyland 1595 of 150 hp (110 kW) per DMBS[4][6]
Cylinder count6 (horizontal)[4]
Power output300 hp (220 kW) (DMBS)[1]
TransmissionMechanical (DMBS)[4]
Train heatingOil burning air heater[4]
  • DD10 mk3 and mk4 (DMBS, one of each)
  • DT9 mk4 and mk5 (DTS, one of each)[4]
Braking system(s)Vacuum[4]
Safety system(s)AWS (DMBS)[4]
Coupling systemScrew[7]
Multiple working Blue Square[1]
Headlight typeFluorescent and tungsten[4]
Track gauge4 ft 8 12 in (1,435 mm)

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).[8]

The Class 121 is Britain's longest serving DMU, operating in passenger service for 57 years until 2017.

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.

Unlike the earlier (but similar) Class 122, 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[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. 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[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 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) 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 and 121029) 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 which had been cascaded from Central Trains. The units have all since been transferred to departmental duties.

Arriva Trains Wales[edit]

In 2006, 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. In March 2013, following further extensive periods out of traffic, 121032 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.

Chiltern Railways[edit]

121020 and 121034 at Bewdley in 2004.

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 Class 165 "Network Turbo" unit. This unit has been fitted with secondary (magnetic) 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 (less extensively) 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.[9]

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, and will be sold.[10]

Fleet details[edit]

Active units[edit]

Remarkably, considering the age of the fleet, the oldest unit (55020) was also the final unit withdrawn alongside 55034, which remained in daily service until 2017, whilst the youngest vehicle (55035) was the first to be dismantled. As of July 2014, 55021 (960021) and 55030 (960013) have also been scrapped.


Class 121s have proved popular for preservation on heritage railways.

Vehicle no. Unit no. Vehicle type Location Notes
Original Departmental
56287 L211 DTS Colne Valley Railway
56289 L263 DTS East Lancashire Railway
55020 977722 L120 DMBS Bodmin and Wenford Railway[11] Departmental unit 960002 (1992–2002) then returned to passenger use until 2017[12]
55022 977873 L122 DMBS Southall Railway Centre Departmental unit 960014[13]
55023 L123 DMBS Chinnor and Princes Risborough Railway
55024 977858 L124 DMBS Returns to traffic 30 March 2019
55025 977859 L125 DMBS MoD Long Marston Stored for use on the Bristol and North Somerset Railway[14]
55026 977824 T005 DMBS Preserved at Swansea Vale Railway but scrapped in 2009[15]
55027 977975 P127 DMBS Ecclesbourne Valley Railway Previously Severn Tunnel emergency train 960302
55028 977860 L128 DMBS Swanage Railway
55029 977968 L129 DMBS Rushden, Higham and Wellingborough Railway Damaged by fire in May 2015.[16]
55031 977976 T002 DMBS Ecclesbourne Valley Railway Previously Severn Tunnel emergency train 960303
55032 T003 DMBS Wensleydale Railway
55033 977826 T004 DMBS Colne Valley Railway
55034 977828 T005 DMBS Chiltern Railways Latterly operated by Chiltern Railways as 121034

Order 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

Model railways[edit]

In 2006 Hornby Railways launched its first version of the BR Class 121 in OO gauge.[17] Since 2017 Hornby have produced a basic representation of the prototype as part of their Railroad range in BR Class 121 in BR Green.


  1. ^ a b c d e f g "Class 121, 149". The Railway Centre. Archived from the original on 9 March 2005.
  2. ^ Marsden 1983, p. 88
  3. ^ a b Fox 1987, pp. 30–31
  4. ^ a b c d e f g h i j k l m n o Vehicle Diagram Book No. 220 for Diesel Multiple Unit Trains (Railcars) (PDF). Barrowmore MRG. Derby: British Railways Board. 1982. DQ221, DS201.
  5. ^ a b Fox & Hughes 1994, p. 12
  6. ^ Marsden 2011, p. 98
  7. ^ "Mechanical And Electrical Coupling Index". Rail Safety and Standards Board. Archived from the original on 21 December 2013. Retrieved 20 December 2010.
  8. ^ "Retirement of the Class 121 "Bubble Car" – Last Day of Service on Chiltern Railways". Chiltern Railways. 9 May 2017. Retrieved 20 May 2017.
  9. ^ Miller, Mark (18 May 2011). "Chiltern's latest trains are 50 years apart". RAIL (670): 11.
  10. ^ "The end is nigh for Chiltern's 'Bubble cars'". Rail Magazine. 10 April 2017. Retrieved 4 May 2017.
  11. ^ "55020". Retrieved 13 April 2018.
  12. ^ "55020 Details". Retrieved 13 April 2018.
  13. ^ "55022 Details". Retrieved 13 April 2018.
  14. ^ "55025".
  15. ^ "55026". Retrieved 13 April 2018.
  16. ^ "BR 55029 Class 121 Single Unit DMU: Driving Motor Brake 2nd built 1960".
  17. ^ "Hornby BR Class 121". Hornby Railways Collector Guide. Retrieved 2 February 2020.


  • Fox, Peter (1987). Multiple Unit Pocket Book. British Railways Pocket Book No.2 (Summer/Autumn 1987 ed.). Platform 5 Publishing Ltd. ISBN 0906579740. OCLC 613347580.CS1 maint: ref=harv (link)
  • Fox, Peter; Hughes, Barry (1994). DMUs & Channel Tunnel Stock. British Railways Pocket Book No.3 (7th ed.). Platform 5. ISBN 9781872524597.CS1 maint: ref=harv (link)
  • Marsden, Colin J. (1983). Motive Power Combined Volume. Ian Allan. ISBN 0711012849.CS1 maint: ref=harv (link)
  • Marsden, Colin J. (2011). Traction Recognition (2nd ed.). Ian Allan. ISBN 9780711034945.CS1 maint: ref=harv (link)

External links[edit]