The British Rail Class 357 "Electrostar" alternating current (AC) electric multiple units (EMUs) were built by ADtranz (now owned by Bombardier Transportation) at their Litchurch Lane Works in Derby, England, in two batches from 1999 to 2002 at a cost of approximately £292 million. They were the first member of the Electrostar family, which also includes Classes 375, 376, 377, 378, 379 and 387, and is the most numerous type of EMU built in the post-privatisation period of Britain's railways.