The British Rail Class 91 is a high-speed electric locomotive, which produces power of ; it was ordered as a component of the East Coast Main Line modernisation and electrification programme of the late 1980s. The Class 91s were given the auxiliary name of InterCity 225 to indicate their envisaged top speed of ; they were also referred to as Electras by British Rail during their development and throughout the electrification of the East Coast Main Line. The other end of the InterCity 225 train set is formed of a Mark 4 Driving Van Trailer, built with a similar body shell to the Class 91 locomotives but with only one driving cab. The locomotive body shells are of all-steel construction. Unusually, the motors are body mounted and drive bogie-mounted gearboxes via cardan shafts; this reduces the unsprung mass and hence track wear at high speeds. The locomotive also features an underslung transformer, therefore the body is relatively empty compared to contemporary electric locomotives. Much of the engineering specification for the locomotive was derived from the research and operational experience of the APT-P. History Background The origins of the Class 91 are closely associated with the East Coast Main Line (ECML) that it has been primarily operated upon. During the 1950s, British Rail had considered electrification of the ECML to be of equal importance to the West Coast Main Line (WCML), but various political factors led to the envisioned electrification programme being delayed for decades; as an alternative, high-speed diesel traction, including the Deltic and InterCity 125, was introduced upon the route during the 1960s and 1970s. During the 1970s, a working group of British Rail and Department of Transport officials determined that, out of all options for further electrification, the ECML represented the best value by far. Its in-house forecasts determined that increases in revenue and considerable reductions in energy and maintenance costs would occur by electrifying the line. Accordingly, between 1976 and 1991, the ECML was electrified with overhead lines. The electrification was installed in two phases: The first phase between London (King's Cross) and (including the Hertford Loop Line) was carried out between 1976 and 1978 as the Great Northern Suburban Electrification Project, using Mk. 3A equipment, covering in total. In 1984, the second phase commenced to electrify the northern section to Edinburgh and Leeds. During the late 1980s, the programme was claimed to be the longest construction