The British Rail Class 321 alternating current (AC) electric multiple units (EMU) were built by BREL York in three batches from 1988 to 1991. The design was successful and led to the development of the similar Class 320 and Class 322 units for use by Strathclyde PTE and Stansted Express (now used by Northern Rail) respectively. The British Rail Mark 3 bodyshell design was also used for construction of the Class 456 direct current (DC) units.