| British Rail Class 121 Bubble Car | |
|---|---|
 A Class 121 atReading in 1992 | |
 Interior of 121032 | |
| In service | May 1960 – present |
| Manufacturer | Pressed Steel Company[1] |
| Orderno. | |
| Family name | First generation |
| Replaced | Steam locomotives and carriages GWR railcars |
| Constructed | 1960–1961 |
| Entered service | 1960 |
| Scrapped | 1978–2011 |
| Number built |
|
| Number preserved |
|
| Number scrapped |
|
| Formation |
|
| Diagram | |
| Fleet numbers |
|
| Capacity |
|
| Operators | British Railways Network SouthEast Regional Railways Silverlink Railtrack Network Rail Arriva Trains Wales Chiltern Railways |
| Depots | |
| Lines served | |
| Specifications | |
| Car body construction | Steel[1] |
| Car length | 64 ft 6 in (19,660 mm) (over body)[4] |
| Width | 9 ft 0 in (2,740 mm) (over body)[4] |
| Height | 12 ft4+1⁄2 in (3,772 mm)[4] |
| Doors | Slam[1] |
| Articulated sections | Single car (2) |
| Wheelbase |
|
| Maximum speed | 70 mph (113 km/h)[4] |
| Weight |
|
| Prime mover(s) | |
| Engine type | 11.1-litrenaturally aspiratedDiesel |
| Cylinder count | 6 (horizontal) per engine[4] |
| Power output | 300 hp (220 kW) (DMBS)[1] |
| Transmission | Mechanical (DMBS)[4] |
| HVAC | Oil burning air heater[4] |
| Bogies |
|
| Braking system(s) | Vacuum[4] |
| Safety system(s) | AWS (DMBS)[4] |
| Coupling system | Screw[11] |
| Multiple working | ■ Blue Square[1] |
| Headlight type | Fluorescent and tungsten[4] |
| Track gauge | 4 ft 8+1⁄2 in (1,435 mm) |
TheBritish Rail Class 121 is a single-car double-endeddiesel 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 someenthusiasts (a nickname endorsed and made official by final passenger service operatorChiltern Railways).[12]
The Class 121 is Britain's longest serving DMU, operating in passenger service for 57 years until 2017.
The Class 121 vehicles were introduced in 1960 for use on theWestern Region of British Rail. They were used on various lightly used branch lines inCornwall including theLooe branch, the branch lines off the main line in the Thames Valley including theGreenford branch line, theBridport branch line (closed 1975), and theSevern Beach line in Bristol. In 1978 all of the units were still allocated to Western Region depots.[13]
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.
Upon privatisation of Britain's railways, the Class 121 fleet was only operated by one passenger company, namelySilverlink, with several more units in departmental duties withRailtrack. SinceChiltern Railways retired its final two units in May 2017, there have been no examples left in revenue-making service.
As of 2026, two units are owned byLocomotive Services Limited:
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Silverlink inherited a small fleet of four "Bubble Cars". They were mainly used on theMarston Vale line fromBletchley toBedford, as well as non-electrified lines in North London, such as theGospel Oak to Barking line. The units replaced the previous fleet ofClass 108 andClass 115 units, and were supplemented with a fleet ofClass 117 units cascaded from Thames Valley services.
The four units, nos. 55023/27/29/31, were based atBletchley depot, where staff repainted set L123 (55023) into its original British Railways green livery. The others remained in obsoleteNetwork SouthEast livery. In 1996, set L123 was withdrawn from traffic, and the other three were hired toGreat Eastern for use on theSudbury branch. 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 withdrawnClass 117 units.
The units were replaced on Silverlink duties in 2001 byClass 150Sprinter units which had been cascaded fromCentral Trains. The units have all since been transferred to departmental duties.
In 2003, Chiltern Railways reintroduced"Heritage" diesel multiple units on itsAylesbury to Princes Risborough shuttle service. For this purpose, unit 121020 was purchased fromNetwork Rail, and heavily refurbished to allow it to operate passenger services. It was repainted intoChiltern Railways blue livery. The introduction of this unit allowed the release of aClass 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 allMark 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 theBirmingham Railway Museum. It is painted in BR Green livery and was also used on Aylesbury to Princes Risborough services, when required.[19]
Both units were additionally used on shuttles, as required, between Aylesbury and Quainton Road stations, on the occasion of events at theBuckinghamshire Railway Centre.
In March 2013, Chiltern Railways acquired 121032 fromArriva 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 given the age of the units, which were more than 50 years old. They were subsequently sold.[20]
Class 121s have proved popular for preservation onheritage railways.
| Vehicle no. | Unit no. | Vehicle type | Location | Notes | |
|---|---|---|---|---|---|
| Original | Departmental | ||||
| 56287 | – | L211 | DTS | Epping Ongar Railway[21] | |
| 56289 | – | L263 | DTS | East Lancashire Railway | |
| 55020 | 977722 | L120 | DMBS | Bodmin and Wenford Railway[22] | Departmental unit 960002 (1992–2002) then returned to passenger use until 2017[23] |
| 55023 | – | L123 | DMBS | Chinnor and Princes Risborough Railway | |
| 55024 | 977858 | L124 | DMBS | Returned to traffic 30 March 2019 | |
| 55025 | 977859 | L125 | DMBS | Vale of Berkeley Railway | Unit sorted pending restoration[24] |
| 55027 | 977975 | P127 | DMBS | Ecclesbourne Valley Railway | PreviouslySevern 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.[25] |
| 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 | Only bubble car to have carried Midline livery, when operating in the West Midlands PTE area. |
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| 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 |
AOO gauge model of the Class 121 was produced byLima in 1998. The model was produced as unit 55035 in all-over rail blue livery and unit 55027 in Network SouthEast livery.[26] In 2006,Hornby Railways launched its first version of the BR Class 121 in OO gauge.[27] This model was produced in BR blue and BR green, using the Lima body mouldings that Hornby had acquired.[28] Since 2017 Hornby have produced a basic representation of the prototype as part of their Railroad range in BR Green.
