66097 on 4R13 Cottam Power Station to Immingham empty Coal Hoppers at Knabbs Bridge 27th February 2013

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 Series 66 (JT42CWR).
On the privatisation of British Rail's freight operations in 1996, Wisconsin Central Transportation Systems under the control of Ed Burkhardt bought a number of the newly privatised rail freight companies: Transrail; Mainline; Loadhaul; and later Railfreight Distribution and Rail Express Systems. Controlling 93% of UK rail freight, after a public relations exercise involving the input of the general public, the company was named English Welsh & Scottish.

EWS inherited a fleet of 1,600 mainly diesel locomotives, with an average age of over 30 years; 300 had been cannibalised for spares. Typical of the fleet, the 2580 hp Class 47s needed a major overhaul every seven years, costing £400,000; yet had an average daily availability of less than 65% with only 16 days between major failures. To enable it to offer its stated lower pricing to customers, EWS needed to reduce operating costs and raise availability.
After reviewing the existing privately commissioned Class 59, which was more powerful, highly reliable and with lower operating costs, EWS approached its builder Electro-Motive Diesel (EMD), then a division of General Motors. EMD offered their JT42CWR model, which had the same loading gauge-passing bodyshell as the Class 59. The engine and traction motors were different models to enable higher speeds, and the Class 66s incorporated General Motors' version of a "self-steering bogie" ("radial truck", in American usage), designed to reduce track wear and increase adhesion on curves.

Placing what was termed as "the biggest British loco order since steam days", EWS placed an order for 250 units to be built at the EMD plant in London, Ontario, Canada.The EMD 710 12-cylinder diesel engine is a development of one used over 20 years, whilst the EM2000 control equipment is the same as that used on Irish Railways IE 201 Class. EWS reduced the locomotive's time into operation through specifying cab systems laid out like the Class 59, whilst increasing availability with a fuel tank of 8,180 litres (1,800 imp gal; 2,160 US gal) capacity, compared to 3,470 litres (760 imp gal; 920 US gal) on a standard Class 47.
The first locomotive shipped to the UK arrived at Immingham in June 1998, taken to Derby for testing. The second was taken to AAR's Pueblo Test Centre for endurance testing, before shipping to the UK. The locomotives then shipped at a rate of 11 per month into the UK via Newport Docks, until the order was completed in December 2001. After unloading, EWS engineers then simply took off the tarpaulin, unblocked the suspension, and finally as each was shipped with water and fuel, hooked up the batteries, before starting the engine and handing the locomotive into service.

Each locomotive is specified and guaranteed to 95% availability, aiming for a minimum of 180 days mean time between failures. It is designed to cover 1·6million km between major rebuilds, equivalent to 18 years' service, with each major rebuild costed at £200,000.

The initial classification was as Class 61, then they were subsequently given the Class 66 designation in the British classification system (TOPS). In 1998 Freightliner placed an order for locomotives. They were followed by GB Railfreight, and then Direct Rail Services.

Although sometimes unpopular with many rail enthusiasts, due to their ubiquity and having caused the displacement of several older types of (mostly) British built locomotives, their high reliability has helped rail freight to remain competitive. Rail enthusiasts call them "sheds".