 The line was originally built with two tracks, but growing traffic meant that several stretches were widened to four tracks between 1871 and 1909.The line was electrified on the 25 kV AC system during the 1960s, in the wake of the 1955 British Rail modernisation plan.Prior to this work being carried out, the West Coast Main Line had four tracks between London and Rugby, comprising a "fast line" and a "slow line" in each direction (the slow lines diverting via the Northampton Loop Line).