BR mixed traffic type 3 locos built designed by English Electric at Vulcan Foundry and Robert Stevenson and Hawthorns at Darlington between 1960 and 1965. Originally numbered D6700-D6999 and D6600-D6608, renumbered into the TOPS series, originally in one block as 37001 to 37309. Modifications and rebuilding saw the following sub classes come into use. Class 37/3 (37310 to 37326) 1986 to 1989, 12 locos allocated to Motherwell with strengthened couplings and modified brake blocks for Ravenscraig workings. Class 37/3 (37330 to 37384) Locos modified and renumbered from 1994 with CP7 bogies and additional fuel capacity. Class 37/4 (37401-37431) Locos modified and renumbered from 1984 with ETH fitted, CP7 bogies and main generator replaced with an alternator. Class 37/5 (37501 to 37521, 37667 to 37699) Refurbished examples from 1986, CP7 bogies and main generator replaced with an alternator. Class 37/6 (37601 to 37612) Class 37/5 modified for the abortive Nightstar Project from 1996. Class 37/7 (37701 to 37719, 37796 to 37803, 37883 to 37899) Refurbished examples from 1986, CP7 bogies and main generator replaced with an alternator, ballast weights also fitted for extra adhesion when working heavy freight trains. Class 37/9 (37901 to 37906) Refurbished examples from 1986, fitted as testbeds for Mirrlees or Ruston engines together with additional ballast weights. Most examples now withdrawn and scrapped with a small number in use with Colas, DRS and West Coast Railway Company with more than 30 preserved.