the Class 37s found use on both passenger and freight workings, being one of the mainstays of the BR fleet. While freight use was widespread, passenger work was predominantly in the Eastern Region with use on London - Norwich and Cambridge/King's Lynn services, the latter through to the 1980s and in West Wales on services west of Swansea to Fishguard Harbour, Milford Haven and Pembroke Dock. Darnall's (later Tinsley's) 37s were used on various passenger services in the Sheffield area, notably the Pullman service to London King's Cross. Hull Dairycoates' 37s were used on local stopping services as well as on King's Cross portions down to Doncaster. In the early 1980s, the remaining steam-heat capable locomotives gravitated to Scotland, eventually replacing both Class 27s on the West Highland Line and Class 26s on the Far North and Kyle of Lochalsh lines. However, their use became difficult once ETH-fitted Mark 2 and Mark 3 coaches moved north. The sleeper trains were particularly troublesome. To this end, steam-heat 37/0s were coupled to ETHELs (Electric Train Heating Ex-Locomotive - essentially Class 25s with isolated traction motors that provided Electric Train Heating) which provided the necessary power to run them. Class 37s were subject to several modifications during their time in service. One short-lived experiment involved 37 175 receiving CP5 'self-steering bogies' designed to reduce excess wear and noise on the tightly-curved West Highland line to Fort William and Mallaig. This however, proved too expensive to be practicable. Some Inverness allocated Class 37s received 'car lights'; these were essentially spotlights that made them more visible on the sharply curved Scottish branches, especially to users of level crossings. These were eventually either removed or superseded by the modern 'sealed beam' lights that became compulsory from 1993. Other classes fitted with these or similar lights were Class 24s, Class 26s, Class 47s and a solitary Class 86 86 225. The other notable use was of nominally freight locomotives to Aberystwyth over the Cambrian line, in this case taking over from Class 25s on the holiday trains. From 1985, the 37/4 subclass took over on the Scottish lines although the use of original 37/0s continued in the summer when train supply was not required. The Welsh allocation in turn took over from the 37/0s on the Cambrian as well as from Class 33s on Crewe–Cardiff workings. They were also used on the Bristol–Cardiff/Swansea leg of services from