Revealed: London's secret underground railway system - CNN Style
How many cities have underground in UK? - Quora
How many cities have underground in UK? - Quora
World's oldest metro systems - Railway Technology
What the Subway Is Like in 16 Different Cities.
London Underground: The UK cities with an underground Tube system you might not know about - MyLondon
London Underground: The UK cities with an underground Tube system you might not know about - MyLondon
Bristol could get underground railway to beat road congestion
A tale of two metros: how the London tube beat the New York subway | Underground cities | The Guardian
London Underground: The UK cities with an underground Tube system you might not know about - MyLondon
The last metro? Crossrail and the uncertain future of urban transit | Financial Times
UK Transport System and How It Compares With the USA - WanderWisdom
Rail transport in Great Britain - Wikipedia
London Underground: The UK cities with an underground Tube system you might not know about - MyLondon
British trains 'up to four times slower outside the south-east' | Rail transport | The Guardian
List of metro systems - Wikipedia
London Underground | subway, London, England, United Kingdom | Britannica
End London-centric approach and invest £140bn in 11 underground systems, Scape say | Infrastructure Intelligence
Longest Metro And Subway Rail Systems In The World - WorldAtlas
Why do so few cities in the UK have metro systems? - Quora
Why do so few cities in the UK have metro systems? - Quora
London Underground: The UK cities with an underground Tube system you might not know about - MyLondon
The World's Oldest Metro Systems - WorldAtlas
