(CNN)Here's some background information about the London Underground, also known as "The Tube," the oldest subway or metro transit system in the world. There are currently 270 stations open and 249 miles (402 km) of active lines. There are a number of old stations and tunnels that are now closed.
January 9, 1863 - The London Underground begins running on the Metropolitan Railway. Forty thousand passengers ride the Underground on the first day. This route is now served by the Circle Line and the Hammersmith & City Line.
1863 - Metropolitan Line opens (purple line on the map).
1864 - Hammersmith & City Line opens (pink line on the map).
1868 - District Line opens (green line on the map).
1869 - East London Line opens (orange line on the map).
1884 - Circle Line opens (yellow line on the map).
1890 - Northern Line opens (black line on the map).
1898 - Waterloo & City Line opens (teal on the map).
1900 - Central Line opens (red line on the map).
1906 - Bakerloo Line opens (brown line on the map). Piccadilly Line opens (dark blue on the map).
1911 - The first escalators are installed, at Earl's Court station.
1913 - First appearance of the circle and horizontal bar symbol.
1929 - The last manually operated doors on tube trains are replaced by air-operated doors.
1969 - Victoria Line opens (light blue on the map).
1979 - Jubilee Line opens (silver line on the map).
November 18, 1987 - A fire at King's Cross station is caused by a burning match falling onto a wooden-tread escalator panel. Thirty-one people die in the fire.
July 7, 2005 - Four suicide bombers detonate themselves aboard three Tube trains and a bus during morning rush hour, killing 52 people and injuring around 700.
July 21, 2005 - Four bombing incidents in London take place almost simultaneously, three on subway trains, one on a bus. Three small devices explode at three separate Underground subway stations, two weeks after the July 7 terror attacks. Small blasts occur at Warren Street, Oval and Shepherd's Bush stations. Scotland Yard also responds to an "incident" on a bus at Hackney Road and Columbia Road in east London. There are no casualties.
2007- For the first time, the Tube carries one billion passengers in a year.
2013 - The 150th anniversary.
July 8, 2015 - A 24-hour strike begins 6 p.m. local time. The strike action is called after unions and the London Underground fail to agree on a pay deal over a new "Night Tube" service due to start in September.