The Class 47 was produced in large numbers from 1962 to replace the last steam locomotives and therefore had to have an axle load of less than 20 tons. They were powered by a Sulzer twelve-cylinder engine with two cylinder banks. The initial 2,750 hp was soon reduced to 2,580 hp to improve reliability. Initially, most of the locomotives were given steam boilers, which were later replaced with electric train heating. This resulted in a large number of sub-variants that are used for passenger and freight trains and have a maximum speed of 75, 95 or 100 mph. A total of 33 locomotives were converted to Class 57 from 1998. The number of locomotives in use that have not been converted has now fallen significantly.