The Class 40 locomotive was constructed by English Electric from 1958 and was a direct development of the prototype locomotive 10203 (and to a lesser extent 10000/1 and 10201/2) which was built as a prototype 5 years previously. The locomotive weighs a total of 133 tons fully fueled (710 gallons main ,200 gallons boiler) (3.4 tons) and 800 gallons of boiler water (3.6 tons). The engine fitted is an English Electric English Electric 16SVT MkII with a rated power of 2000 bhp. The typical specific fuel consumption (sfc) at 2000 bhp is 0.42 lb per hour/hp. At a typical 75% load factor the sfc is 0.37. On a typical run from London Kings Cross to Edinburgh approximately 1.6 tons of fuel (425 gallons) would be consumed.
The locomotive has 6 powered and 2 non-powered axles . The powered axles support 112 tons and the non-powered axles 21 tons. At 22.5% adhesion this gives a maximum tractive effort of 52000 lbf. The engine has a nominal rating of 2000 bhp and has a setting range of 1950-2050 bhp giving a variance of +/- 2.5%. The design of the governor is such that in virtually conditions it is the lower limit (1950 bhp) that is available and this is the output that the tractive effort curve is based on. The main generator is dc driven and has 3 stages of field weakening (27 mph, 38 mph and 56 mph) however the setting up of these field diverts requires a degree of skill and it is not unknown for some variation to occur. The engine like most of its generation suffers a fairly rapid fall off in performance after major overhauls and it was not uncommon to see 150 bhp shortfalls especially in those locomotives operating on largely freight diagrams. The rate of drop off in performance was reduced over time as various engine modifications were introduced however from about 1980 large variations were again noticeable as the run down of the locomotives started. At the time of their introduction the locomotives had insufficient cooling capacity and it was common on the long climbs to suffer high temperature which was mitigated by a reduction in throttle. This was rectified by modifications to the cooling circuit and the removal of frost guards. The locomotives were initially fitted with vacuum brakes and steam heating. Over time a number lost their boilers and either air or dual brakes were fitted. Apart from a brief trail with a single unit (D255) no electric train heating (ETH) was fitted. The locomotive had a maximum service speed of 90 mph and due to the relatively low installed power and off-loading above 79 mph it was rare to significantly exceed this speed.