History of the London Underground to determine the cause. Following the incident, a system that stops a train automatically if the driver fails to brake was introduced at dead-ends on the London Underground. The Northern City Line was connected to British Rail tracks at Finsbury Park in the 1970s. The last tube train ran in October 1975, and British Rail services began in 1976. In 1977, the Piccadilly line was extended from Hounslow to Heathrow Airport, and in 1986 a platform serving Terminal 4 opened on a loop line. The Fleet line through central London was first proposed in 1965, taking over the Bakerloo line's
London Underground has expanded to 11 lines, and in 2016–17 carried 1.379 billion passengers, making it the world's 11th busiest metro system. The 11 lines collectively handle approximately 4.8 million passengers a day. The system's first tunnels were built just below the surface, using the cut-and-cover method; later, smaller, roughly circular tunnels—which gave rise to its nickname, the Tube—were dug through at a deeper level. The system has 270 stations and of track. Despite its name, only 45% of the system is underground in tunnels, with much of the network in the outer environs of London being on the surface. In addition,