the british rail class 357 "electrostar" alternating current (ac) electric multiple units (emu) were built by adtranz, derby (now owned by bombardier transportation) from 1999 to 2003 at a cost of approximately £350 million. the electrostar family also includes classes 375, 376, 377 and 378, and is the most numerous type of emu built in the post-privatisation period of britain's railways. these units were built in two batches for lts rail, since rebranded as c2c.