The British class 66, designated JT42CWR by its manufacturer EMD, is a diesel-electric freight locomotive tailored for use in Britain. Compared to the earlier class 59, it is being powered by the more modern 710 series engine. It has two three-axle bogies with self-steering axles. The standard variant features a top speed of 75 mph or 121 km/h and a starting tractive effort of 409 kN. The subclass 66/6 ordered by Freightliner has a reduced gear ratio for a top speed of only 65 mph or 105 km/h, but a starting tractive effort of 467 kN. The dimensions of the body were adjusted for the small British loading gauge. Due to their appearance and especially the shape of the roof, they are being nicknamed “sheds”.

The first 250 orders in the UK came from EWS and they ultimately numbered 455. Soon there were more orders from operators from the European mainland, with the first orders coming from the German HGK and the Swedish TGOJ. Soon they could be found in a large number of countries across Europe and also in Egypt and Gabon. While they could be used efficiently for their operators, drivers often expressed complaints about excessive noise, vibration and heat in the cab. As a result, many orders were delivered as the updated class 77 that addressed these complaints. Using the larger European loading gauges, these have air conditioning units on top of the cabs, better sound proofing and comply with more stringent emissions standards. Additionally, the bogies were upgraded for a starting tractive effort of 450 kN.