The British Rail Class 77, also known as Class EM2, is a class of 1.5 kV DC, Co-Co electric locomotive. They were built by Metropolitan-Vickers in 1953–1954 for use over the Woodhead Line between Manchester and Sheffield. 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. The locomotives were initially numbered 27000-27006 and were painted in BR black livery. Construction took place at Gorton Works, Manchester with electrical equipment supplied by Metropolitan-Vickers. All seven were named after characters from Greek mythology in 1959-1960. They were primarily used for express passenger trains between Manchester Piccadilly and Sheffield Victoria. In 1957, the class was renumbered with the addition of an "E" prefix to the number.