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British Rail Class 121

British Rail Class 121
Bubble Car
A Class 121 at Reading in 1992
Interior of 121032
In service1960–present
ManufacturerPressed Steel Company[1]
Order no.
  • 30518 (DMBS)
  • 30519 (DTS)[2]
Family nameFirst generation
ReplacedSteam locomotives and carriages
GWR railcars
Constructed1960–1961
Entered service1960
Scrapped1978–2011
Number built
  • 16 (DMBS)
  • 10 (DTS)
Number preserved
  • 12 (DMBS)
  • 2 (DTS)
Number scrapped
  • 4 (DMBS)
  • 8 (DTS)
Formation
  • Single car (with trailer)
  • DMBS(+DTS)
Diagram
  • 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]
Capacity
  • 65 (DMBS)
  • 91 (DTS)[4]
OperatorsBritish Railways
Network SouthEast
Regional Railways
Silverlink
Railtrack
Network Rail
Arriva Trains Wales
Chiltern Railways
Depots
Lines served
Specifications
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+1⁄2 in (3,772 mm)[4]
DoorsSlam[1]
Articulated sectionsSingle car (2)
Wheelbase
  • 46 ft 6 in (14.17 m) (bogie centres)
  • 8 ft 6 in (2.59 m) (bogies)[4]
Maximum speed70 mph (113 km/h)[4]
Weight
  • 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][10]
Engine type11.1-litre naturally aspirated Diesel
Cylinder count6 (horizontal) per engine [4]
Power output300 hp (220 kW) (DMBS)[1]
TransmissionMechanical (DMBS)[4]
HVACOil burning air heater[4]
Bogies
  • 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[11]
Multiple working■ Blue Square[1]
Headlight typeFluorescent and tungsten[4]
Track gauge4 ft 8+1⁄2 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 (113 km/h), 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).[12]

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

  1. ^ a b c d e f g "Class 121, 149". The Railway Centre. Archived from the original on 9 March 2005.{{cite web}}: CS1 maint: unfit URL (link)
  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). Derby: British Railways Board. 1982. DQ221, DS201. Archived from the original (PDF) on 21 January 2015. Retrieved 19 February 2016 – via Barrowmore MRG.
  5. ^ a b Fox & Hughes 1994, p. 12
  6. ^ a b Hemming 1978, p. 12.
  7. ^ Heginbotham, Stephen (2010). Cornwall's Railways Remembered. Halsgrove. pp. 51–52. ISBN 978-0-85704-005-3.
  8. ^ "ROLLING STOCK". The North Cornwall Railway. Archived from the original on 11 April 2021. Retrieved 11 April 2021.
  9. ^ Vaughan 1983, p. 106.
  10. ^ Marsden 2011, p. 98
  11. ^ "Mechanical And Electrical Coupling Index". Rail Safety and Standards Board. Archived from the original on 21 December 2013. Retrieved 20 December 2010.
  12. ^ "Retirement of the Class 121 "Bubble Car" – Last Day of Service on Chiltern Railways". Chiltern Railways. 9 May 2017. Archived from the original on 14 April 2018. Retrieved 20 May 2017.
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