The General Motors heavy freight design was introduced in 1998 and, besides becoming the standard British heavy freight locomotive, it is being adopted further afield in Europe. The move towards GM domination started with the introduction of the Class 59 by Foster Yeoman in 1985. Those locomotives were the only ones the company could find to meet their requirements. When the American backed company English Welsh & Scottish (EWS) took over most of the freight business in Britain, they immediately sought to replace the ageing locomotive fleet with highly reliable and efficient machines and turned to General Motors to supply them. 250 of an updated Class 59 were ordered and built in Canada and these became the Class 66. Freightliner, Direct Rail Services (DRS) and GB Railfreight all followed suite.

Few modifications have been made and each locomotive has a combination of couplings. One major change has been the introduction, since 2004, of 'green' locos with low-emission technology. Locomotives so fitted have an extra external door on the 'A' side of the machine.

There are six subclasses resulting from the specific needs of individual customers.

66/0s were supplied to EWS and are numbered 66001-66250.

66/4s are 30 locomotives supplied to DRS.

66/5s are the 99 standard Freightliner locomotives.

66/6s are 25 Freightliner locomotives that have modified gearing.

66/7s are 32 standard GB Railfreight locomotives.

66/9s are the 2 locomotives used for the development of the low-emission technology.