The Class 37 locomotive, originally called the English Electric Type 3, was originally introduced in 1961 with a total of 308 locomotive being constructed.  The tractive effort has been shrouded in mystery for many years with no official test report having been issued.  The often-quoted rail horsepower of 1 250 bhp does not correlate with the quoted maximum tractive effort of 35 000lbf at 13.6 mph which equates to 1 269 rail horsepower. In both cases this looks to be low for a locomotive rated at 1750 brake horsepower at the crankshaft especially with the prototype locomotive 10202 which was also rated at 1 750 bhp having a continuous rail horsepower of 1 381 (14 000 lbf @ 37 mph and 2 1000 lbf @ 24.7 mph after re-gearing).  The difference can however be explained as follows:

1. Auxiliary Generator

The auxiliary generator fitted to locomotive 10202 was rated at 45.9 kW compared to the rating of 105 kW in the case of the Class 37.  This was because of the unit in 10202 being somewhat undersized. It has been assumed that the unit in 10202 was running at full load under test and that the expected load on the Class 37 was 93% leaving sufficient margin for future upgrades. In each case the efficiency of the auxiliary generator has been assumed to be 85%.

1. Main Generator

The main generator fitted to locomotive 10202 was rated at 1 155 kW compared to the rating of 1105 kW for the Class 37.  The rationale for the difference is that with a higher offtake for the auxiliary generator less power is available for the main generator.

1. Traction Motors

The traction motors are very similar between the locomotives and have typical maximum efficiencies of 89% which provides a maximum output of 1318 hp at the rail for the Class 37 and 1387 hp for the prototype locomotive. This still leaves a difference to the 1 269 hp that is shown as the continuous rating for the Class 37 however this is accounted for by the fact that the traction motor efficiency at low speeds is somewhat lower than at high speeds.  Information published indicates that an English Electric traction motor from this period has an efficiency of 85.6% at a rotational speed equivalent to 13.6mph.  This would equate to a rail horsepower of 1 268   which matches the published continuous output.

The Class 37 has 2 field diverts which operate at approximately 26 mph and 49 mph which provides roughly full engine output up to 79 mph after which the available power declines. It has been assumed that the characteristics of the traction motor and generator beyond the unloading point are similar between the prototype and production locomotives.