Seven locomotives of this type were constructed. They represented the first Co-Co type of overhead electric locomotive built for use in the United Kingdom. The design was based on that of the smaller Class EM1, which dated from 1941. Initially, 27 locomotives of this type had been planned, but by the early 1950s, the benefits of using the 25 kV AC system had been demonstrated, which meant that the Woodhead Line would be an isolated electric system. Consequently, the order was cut to just seven locomotives.