The Great Northern & City Railway (GN&CR) was planned to allow electrified trains to run from the Great Northern Railway (GNR, now the East Coast Main Line) at Finsbury Park to the City of London at Moorgate. Despite being built using similar methods to the tube network then under construction, the tunnels were built large enough to take a main-line train, with an internal diameter of , compared with those of the Central London Railway with a diameter less than . For this reason the line was popularly known as the "Big Tube" in its early days. However, the GNR eventually opposed the scheme and cancelled its electrification plans, and the line opened in 1904 with the northern terminus in tunnels underneath Finsbury Park GNR station. It was originally electrified using an unusual fourth-rail system with a conductor rail outside both running rails.