Accordingly, between 1976 and 1991, the ECML was electrified with 25 kV AC overhead lines. The electrification was installed in two phases: The first phase between London (King's Cross) and (including the Hertford Loop Line) was carried out between 1976 and 1978 as the Great Northern Suburban Electrification Project, using Mk.3A equipment, covering 30 miles in total. In 1984, the second phase commenced to electrify the Northern section to Edinburgh and Leeds. During the late 1980s, the programme was claimed to be the longest construction site in the world, spanning more than .