British Rail Class 311

British Rail Class 311
Unit 311104 in blue/grey livery awaiting passengers at Partick
In service	1967 - 1990
Manufacturer	Cravens
Order number	30767 (DTSO) 30768 (MBSO) 30769 (BDTSO)
Built at	Cravens Sheffield
Family name	BR First Generation (Mark 1)
Constructed	1967
Entered service	1967
Number built	19
Formation	3 cars per trainset: DTSO+MBSO+BDTSO
Diagram	ED211 (MBS) EE214 (DTS) EF212 (BDTS)
Design code	AM11
Fleet numbers	311092-311110 (sets) 76403-76421 (DTSO) 62163-62181 (MBSO) 76422-76440 (BDTSO)
Capacity	236S (total) 83S (DTSO) 70S (MBSO) 83S (BDTSO)
Operator(s)	British Rail
Depot(s)	Glasgow Shields Road
Specifications
Car body construction	Steel
Train length	199 ft 6 in (60.81 m)
Car length	63 ft 6 1⁄8 in (19.358 m) (MBSO) 64 ft 0 in (19.498 m) (BDTSO, DTSO)
Width	9 ft 3 in (2.819 m)
Height	12 ft 8 in (3.861 m)
Doors	Double sliding leaf, pneumatic
Wheelbase	46 ft 6 in (14.173 m)
Maximum speed	75 mph (121 km/h)
Weight	129 tonnes (127 long tons; 142 short tons) (total) 34.4 t (33.9 long tons; 37.9 short tons) (DTSO) 56.4 t (55.5 long tons; 62.2 short tons) (MBSO) 38.4 t (37.8 long tons; 42.3 short tons) (BDTSO)
Traction motors	4 × AEI 221 hp (165 kW)
Power output	890 hp (660 kW)
Train heating	Electric
Electric system(s)	25 kV 50 Hz AC OHLE
Current collection method	Pantograph
UIC classification	2'2'+Bo'Bo'+2'2'
Bogies	Gresley ED11 (MBSO) Gresley ET11 (BDTSO, DTSO)
Braking system(s)	Air (Auto/EP)
Safety system(s)	AWS
Coupling system	Buckeye
Multiple working	Within Class and Class 303
Track gauge	1,435 mm (4 ft 8 1⁄2 in) standard gauge

The British Rail Class 311 alternating current (AC) electric multiple units (EMU) were built by Cravens at Sheffield in 1967. They were intended for use on the line from Glasgow Central to Gourock and Wemyss Bay, which was electrified in 1967.

Outwardly, the units were virtually identical to the earlier Class 303 units built in 1960. The interiors were also very similar, although the Class 311s had fluorescent lighting (unlike the Class 303s which were fitted with tungsten filament bulbs). The larger fleet of class 303 units had been built by Pressed Steel at their factory in Linwood, Paisley. By the time the Class 311 were required, Pressed Steel no longer built railway carriages, so Cravens of Sheffield worked to the same original drawings, updated at a few points, to build the trains.

Along with the Class 303s, the wrap-around driving cab windows were replaced with flat, toughened glass in the 1970s to give better protection to drivers in the event of attacks by stone-throwing vandals.

19 units were built, initially classified as AM11 units, and numbered 092-110. This was later changed to Class 311 under the TOPS system, and the units were renumbered 311092-110. Each unit was formed of three carriages; two outer driving trailers, an intermediate motor coach. The technical description of the formation was BDTCOL+MBSO+DTSO. Individual carriages were numbered as follows:

The units were built to operate services on the newly electrified routes from Glasgow Central to Gourock and to Wemyss Bay; now known as the Gourock and Wemyss Bay branches of the Inverclyde Line.