The British Rail Class 20, originally known as an English Electric Type 1, is a class of diesel electric locomotive. 228 locomotives in the class were built by English Electric between 1957 and 1968. The large number was partly to do with the failure of other early designs in the same power range, and reliable locomotives being needed.
The locomotives were originally numbered D8000–D8199 and D8300–D8327. They are known by railway enthusiasts as "Choppers", a name taken from the distinctive beat the engine produces under load which resembles the sound of a helicopter.