The Electro-Motive Diesel (EMD) Class 66 are Co-Co diesel locomotives built by EMD for the European heavy freight market. Designed for use in Great Britain as the British Rail Class 66, a development of the Class 59, they have been adapted and certified for use in other European countries. Outside Europe, 40 locomotives have been sold to Egyptian Railways for passenger operation.

The Class 66 is a type of six-axle diesel-electric freight locomotive developed in part from the Class 59, for use on the railways of the UK. Since its introduction the class has been successful and has been sold to British and other European railway companies. In Continental Europe it is marketed as the EMD Class 66 (JT42CWR).