the british rail class 321 alternating current (ac) electric multiple units (emu) were built by brel york in three batches from 1986–1989. the design was very 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 greater anglia) respectively. the bodyshell design was also used for construction of the class 456 direct current (dc) units. the class 321s were nicknamed 'dusty bins', due to the class number sharing the same name as the game show 3-2-1. (this name has also been given to the diesel multiple unit (dmu) class 153 no. 153321 for the same reason).