By the late 1960s, the
Swindon-built inter-city DMUs operating the - express service were becoming unreliable. In 1970, the decision was made to replace them with locomotive-hauled carriages. So between 1971 and 1973, twenty-four Class 27s were fitted-up with dual (vacuum and air) brakes and reclassified Class 27/1, while 36
Mark 2 carriages (7 brake second opens, 22 open seconds and 7 corridor firsts) swapped their vacuum-operated shoe brakes for air-operated disc brakes and were though-wired with
Blue Star control cables to enable
top and tail push-pull working. It was later decided that as the Mark 2 stock was dual (steam or electric) heated, to convert half the 27/1 fleet to electric train heat, by replacing the train heating boiler with a
Deutz 8-cylinder, air-cooled diesel engine and
alternator. The conversions were then classified as Class 27/2 and were used on one end of the train, with a 27/1 on the other. The very intensive
push-pull service was demanding on the locomotives and reliability started to suffer. The 27/2s appeared prone to fire damage, especially from their
electric train heating alternators. The push-pull sets were replaced in 1979 by single
Class 47/7s at one end of a rake of
Mark 2 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 and subsequently by
Sprinter diesel multiple units (DMUs) in 1987, whilst the remainder were largely used on freight. ==Accidents and incidents==