The Merseyrail third rail network has 68 stations and 75 miles (121 km) of the track, of which 6.5 miles (10.5 km) are underground. Carrying approximately 110,000 passengers each weekday, or 34 million passengers per year, it is one of the most heavily used urban railway networks in the United Kingdom outside London. The first part of the large comprehensive urban network was initially opened in 1977 by merging separate rail lines by constructing new tunnels under Liverpool city centre and Birkenhead. The full 1970s plans for the network were not realised, but the network has been extended on its peripheries with additional extensions proposed. The extensions were created by electrifying existing lines and then transferring the electrified sections into Merseyrail.
