The British Rail Class 317 alternating current electric multiple units (EMUs) were built by BREL York in two batches, from 1981-82 and 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. The Mark 3 bodyshell was also the basis of Class 318, Class 455, and the diesel Class 150.