British Rail Class 121

British Rail Class 121
*Bubble Car*
British Rail Class 121; Bubble Car
	A Class 121 at Reading in 1992
	Interior of 121032
In service	1960–present
Manufacturer	Pressed Steel Company
Order no.	30518 (DMBS); 30519 (DTS);
Family name	First generation
Replaced	Steam locomotives and carriages; GWR railcars
Constructed	1960–1961
Entered service	1960
Scrapped	1978–2011
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); DX201 (DMBS, by 1994);
Fleet numbers	55020–55035 (DMBS); 56280-56289 (DTS, as built); 54280-54289 (DTS, renumbered);
Capacity	65 (DMBS); 91 (DTS);
Operators	British Railways; Network SouthEast; Regional Railways; Silverlink; Railtrack; Network Rail; Arriva Trains Wales; Chiltern Railways
Depots	BR Bath Road; BY Bletchley; LA Laira; RG Reading; TS Tyseley;
Lines served	Aylesbury–Princes Risborough line; Lostwithiel – Fowey; North Cornwall Railway; St Erth - St Ives;
Specifications
Car body construction	Steel
Car length	64 ft 6 in (19,660 mm) (over body)
Width	9 ft 0 in (2,740 mm) (over body)
Height	12 ft 4+1⁄2 in (3,772 mm)
Doors	Slam
Articulated sections	Single car (2)
Wheelbase	46 ft 6 in (14.17 m) (bogie centres); 8 ft 6 in (2.59 m) (bogies);
Maximum speed	70 mph (110 km/h)
Weight	38 t (37 long tons; 42 short tons) (DMBS); 30 t (30 long tons; 33 short tons) (DTS);
Prime mover(s)	2 × AEC of 150 hp (110 kW) per DMBS; or; 2 × Leyland 1595 of 150 hp (110 kW) per DMBS;
Engine type	11.1-litre naturally aspirated Diesel
Cylinder count	6 (horizontal) per engine
Power output	300 hp (220 kW) (DMBS)
Transmission	Mechanical (DMBS)
HVAC	Oil burning air heater
Bogies	DD10 mk3 and mk4 (DMBS, one of each); DT9 mk4 and mk5 (DTS, one of each);
Braking system(s)	Vacuum
Safety system(s)	AWS (DMBS)
Coupling system	Screw
Multiple working	■ Blue Square
Headlight type	Fluorescent and tungsten
Track gauge	4 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, 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).

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

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