BR Class 84

The BR Class 84 locomotives, originally designated AL4, were ten Bo-Bo 25 kV AC electric locomotives built in 1960 by the North British Locomotive Company of Glasgow (mechanical parts) with electrical equipment by GEC Traction. They were the fourth of the five AL-class types ordered for the West Coast Main Line electrification evaluation, and represented North British's only significant involvement in electric traction — the company, Glasgow's historic locomotive builder, would close in 1962 before the evaluation was complete.

The AL4 used GEC electrical equipment rated at approximately 3,200 hp, similar to the AL1 specification but with GEC's own approach to the AC–DC rectification and control system. The ten locomotives entered service on the electrified WCML sections from 1960 and worked express passenger and mixed traffic duties alongside the other AL classes. In service the class was considered less satisfactory than some of its contemporaries, partly attributed to problems with the North British mechanical design, and the locomotives had a somewhat troubled career compared to the English Electric AL3 and the later production Class 86.

The Class 84s were the first of the AL evaluation classes to be largely withdrawn, with most going in 1980–82. One locomotive, 84001, was preserved and is held at Barrow Hill Roundhouse in Derbyshire, representing the sole survivor of the AL4 class and North British's contribution to the WCML electrification programme.