The British Rail Class 317 is an electric multiple unit (EMU) passenger train constructed by British Rail Engineering Limited in two batches, 48 sets being produced in 1981–82 and 24 sets in 1985–87. They were the first of several classes of British Rail EMU to be based on the all-steel Mark 3 bodyshell, departing from the "PEP"-aluminium design which had spawned the earlier Class 313 to Class 315, Class 507 and Class 508. The Mark 3 bodyshell was also the basis of Class 318, Class 455, and the diesel Class 150. The Class 317 uses overhead alternating current electrification. Description Class 317/1 The first batch of 48 units was built in 1981–1982 and was classified as Class 317/1. Units were numbered in the range 317301–317348, and had a maximum speed of . Each unit is composed of four vehicles: two unpowered standard-class vehicles with driver's cabs, an intermediate trailer with both first- and standard-class seating, and an intermediate motor vehicle with second-class seating. The motor vehicle also carries the roof-mounted Stone Faiveley AMBR pantograph. The technical description of the formation of the units is DTSO(A)-MSO-TCO-DTSO(B). Individual vehicles were numbered in the following ranges: DTSO(A): 77000–77047 MSO: 62661–62708 TCO: 71577–71624 DTSO(B): 77048–77095 Bodyshells for the TCO vehicles in this batch were constructed at Derby Litchurch Lane Works, while construction of the other bodyshells and overall assembly was carried out at the Holgate Road Carriage Works in York. The units were built to operate services on the newly electrified London St Pancras to Bedford route and were due to replace the elderly (and unreliable) Class 127 diesel multiple units in October 1982. However, their introduction was delayed until 28 March 1983 due to an industrial dispute with the unions over driver-only operation, and the diesel units lingered on until finally being replaced in mid-1983. Units were delivered in the standard livery of BR blue and grey. In 1986, the route came under control of the Thameslink subsector of newly created Network SouthEast. From 1987, new dual-voltage Class 319 units were introduced on the route, allowing the creation of a new cross-London service, from Bedford to Brighton, via Farringdon and City Thameslink station. The Class 317 units were displaced to outer-suburban services on the WCML out of London Euston to Milton Keynes and Northampton. They replaced the slam-door Class 310 units, which subsequently transferred to the London, Tilbury and Southend railway. However, the