The British Rail Class 91 is a class of high-speed, 4,830 kW (6,480 hp) electric locomotives 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 225 km/h (140 mph), and 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. 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 so that 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.