Original allocations were D5347 - 5369 to Glasgow Eastfield, D5370 - 5378 to Thornaby and D5379 - 5415 to London Cricklewood for Tilbury Boat trains and Cross-London freight services. In the period September to December 1963, some of the Cricklewood allocation were transferred to Leicester and in December 1965 the Thornaby allocation was also nominally transferred to Leicester to join them. Traffic changes combined with reallocation of Class 25s led to the gradual transfer of the Leicester and Cricklewood locomotives to Scotland during 1969 thus concentrating the whole class within Scotland and being part of the replacement fleet that allowed the withdrawal of the poorly performing Clayton Class 17 locomotives from traffic. For many years they were extensively used on the West Highland Line. By September 1986, the final vacuum brake only locos had been withdrawn, regular passenger services had ceased and only 21 locos remained, allocated entirely to Eastfield depot. A mass withdrawal in July 1987 due to the presence of blue asbestos left 27008 as the last in service. Its final working was on 13 August and the loco was officially withdrawn on 19 August 1987. The Class 27s were actually outlived by the older Class 26s, whose less powerful engines were more reliable.
By the late 1960s, the Swindon-built Inter City DMUs operating the Edinburgh Waverley - Glasgow Queen Street express service were becoming unreliable. They were replaced in 1971 by specially adapted Class 27s fitted with push-pull equipment to “top and tail” rakes of six Mark 2 carriages which were specially fitted with air operated disc brakes. At one end would be a Class 27/1, at the other a Class 27/2 (which provided train heating). The very intensive 90 mph (140 km/h) “push-pull” service was demanding on the locomotives and reliability started to suffer. The 27/2s, especially, appeared prone to fire damage, especially from their electric train heatingalternators. The push-pull sets were replaced in 1980 by single Class 47/7s at one end of a rake of Mark 3 carriages and a DBSO. The Class 27/1s and 27/2s were then renumbered to 27/0 and could often be found on Edinburgh-Dundee semi-fast passenger services, until their replacement, briefly by class 101 and then Class 150 Sprinter DMUs in 1987, whilst the remainder were largely used on freight.