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 . They were the first member of the Electrostar family , which 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 .