List of British Rail electric multiple unit classes
AC units operate off 25 kV alternating current (AC) from overhead wires. Where clearances for the overhead wires on the Great Eastern Main Line and London, Tilbury and Southend railway routes were below standard, a reduced voltage of 6.25 kV AC was used. The Midland Railway units used 6.6 kV AC. Under the computer numbering, AC units (including mixed-voltage units that can also work off a DC supply) were given a class in the range 300-399.