As time has gone on, there are other lines that have been added and removed from the Underground Map:

City & South London Railway was built with tunnels of only 10.5 ft (3.20 m) diameter. Enlarged for Northern line to 12.0 ft (3.66 m)

Central line with tunnels of 11 ft 8 1⁄4 in (3.56 m), increased on curves, reduced to 11 ft 6 in (3.51 m) near to stations. This makes Central line trains unique on the London
Underground system because, although the loading gauge of the rolling stock is the same as the other 'tube' lines, the smaller size of the tunnel requires that the positive
conductor rail is 1.6 in (41 mm) higher than on all other lines.

A Parliamentary committee headed by James Stansfeld then reported on 23 May 1892, "The evidence submitted to the Committee on the question of the diameter of the
underground tubes containing the railways has been distinctly in favour of a minimum diameter of 11 ft. 6 in." After that, all tube lines were at least that size.[27]

Piccadilly line with tunnels of 12.0 ft (3.66 m)

Victoria line with tunnels of 12.5 ft (3.81 m); enlarged to reduce air friction.

A brief history of the Underground

From its creation to the present day, London Underground 150-year history has been an eventful one.

London Underground milestones

Constructed by Sir Marc Brunel and his son Isambard, the Thames Tunnel opens

On 10 January, The Metropolitan Railway opens the world's first underground railway, between Paddington (then called Bishop's Road) and Farringdon Street

The first section of the Metropolitan District Railway, from South Kensington to Westminster (now part of the District and Circle lines), opens

The first steam trains travel through the Brunel's' Thames Tunnel

Running from the Tower of London to Bermondsey, the first Tube tunnel opens

The Circle line is completed

On 18 December, The City and South London Railway opens the world's first deep-level electric railway. It runs from King William Street in the City of London,
under the River Thames, to Stockwell

The Prince of Wales opens the Central London Railway from Shepherd's Bush to Bank (the 'Twopenny Tube'). This is now part of the Central line

The Underground Electric Railway Company of London (known as the Underground Group) is formed. By the start of WWI, mergers had brought all lines -
except the Metropolitan line

District and Circle lines become electrified

Baker Street & Waterloo Railway (now part of the Bakerloo line) opens and runs from Baker Street to Kennington Road (now Lambeth North). Great Northern,
Piccadilly & Brompton Railway (now part of the Piccadilly line) opens between Hammersmith and Finsbury Park

Charing Cross, Euston & Hampstead Railway (now part of the Northern line) opens and runs from Charing Cross to Golders Green and Highgate (now Archway).
Albert Stanley (later Lord Ashfield) is appointed General Manager of the Underground Electric Railway Company of London Limited

The name 'Underground' makes its first appearance in stations, and the first electronic ticket-issuing machine is introduced. This year also sees the first appearance
of the famous roundel symbol

London's first escalators are installed at Earl's Court station

The last manually-operated doors on Tube trains are replaced by air-operated doors

The Underground Group and the Metropolitan Railway become part of the London Passenger Transport Board, taking control of all the Capital's railway, bus,
tram, trolleybus and coach services

Harry Beck presents the first diagram of the Underground map

Between September 1940 and May 1945, most Tube station platforms are used as air raid shelters. Some, like the Piccadilly line, Holborn - Aldwych branch,
are closed to store British Museum treasures

The London Passenger Transport Board was nationalised and now becomes the London Transport Executive

The first aluminium train enters service on the District line

Sees the end of the steam and electric locomotive haulage of London Transport passenger trains

The London Transport Executive becomes the London Transport Board, reporting directly to the Minister of Transport

The Queen opens the Victoria line

The London Transport Executive takes over the Underground and the Greater London area bus network, reporting to Greater London Council

The last steam shunting and freight locomotive is withdrawn from service

The Victoria line extends to Brixton

A fatal accident on the Northern line at Moorgate kills 43 people. New safety measures were introduced

The Queen opens Heathrow Central station (Terminals 1 2 3) on the Piccadilly line

The Prince of Wales opens the Jubilee line

A museum about the birthplace of modern urban transportation, called Brunel Engine House, opens to the public

Dot matrix train destination indicators introduced on platforms.

The Hammersmith & City and the Circle lines convert to one-person operation

The Piccadilly line is extended to serve Heathrow Terminal 4

A tragic fire at King's Cross station kills 31 people

First completely painted train enters service

New safety and fire regulations are introduced following the Fennell Report into the King's Cross fire

The London Underground Customer Charter is launched

Reconstruction work on Angel station ended

Work started on the extended Jubilee line from Green Park to Stratford

Penalty fares are introduced

London Underground takes over the Waterloo & City line and responsibility for the stations on the Wimbledon branch of the District line from Putney Bridge
to Wimbledon Park

Aldwych station, and the Central line branch from Epping to Ongar closes

London Underground is restructured in preparation for Public Private Partnership

The extended Jubilee line opens, offering through services from Stanmore to Stratford

The Oyster card is introduced

Busking is legalised

52 people are killed in bomb attacks on three Tube trains and a bus on 7 July