later changed to English Electric 12SVT diesel
|Power:||1, 470 hp (1, 100 kW)|
The original Mirrlees JVS12T engines gave BR a lot of trouble as they began to develop metal fatigue after just a few years service. It is thought that uprating the engines may have been to blame as the originally rated engines did not develop as many faults. The original pilot-scheme locomotives were rated at 1, 250hp but later production batch locomotives were uprated to 1, 365 hp and beyond. One was even uprated to 2, 000hp and became a Type 4!
Eventually the decision was made to give the entire class more reliable English Electric powerplants instead in the mid 1960s  and so they became the Class 31. The Class 31 has the unusual A1A-A1A wheel arrangement which means the middle axle in each bogie is unpowered.
As true mixed traffic locomotives Class 31s could be found on regional and secondary passenger services, freights and engineering trains and have survived into the privatised era. Nowadays they are mainly seen on spot hires and engineering trains . Less than a handful remain in mainline service but 26 have been preserved.
|31/0||Original pilot-scheme batch of 20, non-standard control equipment|
|31/1||Main production variant|
|31/4||With Electric Train Heating (ETH)|
|31/5||Former 31/4s with ETH isolated|
|31/6||Wired for ETH but without equipment|
|Preserved 5580 at Kidderminster SVR|
|Preserved 31 270 in Regional Railways livery at Rowsley South|
|Network Rail 31 233 at Derby|
|DCR 31 452 at Bewdley|
|D5581 at Chinnor showing the original BR green livery|
|Although numbered 31 018 this was the first of the class built, preserved at the NRM|
 Brian Haresnape, Production Diesel-Electrics Types 1-3 (Ian Allan, 1983) p. 48
 Colin J Marsden, Traction Recognition (Second Edition) (Ian Allan, 2009) p. 20