Four British cities have underground railway systems: London, Liverpool, Newcastle and Glasgow.
The London Underground is the biggest by far and is the oldest in the world. It dates from 1863 when the Metropolitan Railway opened the line from Paddington to Farringdon.
The Mersey Railway opened in 1886. It originally had underground stations at Liverpool Central, Liverpool James Street and Birkenhead Hamilton Square, plus four surface stations in Birkenhead.
The Loop and Link scheme was completed in 1977. It consists of two sections of tunnel under Liverpool with stations at Moorfields and Lime Street.
The North British Railway opened the Glasgow City and District Line in 1886, and the Caledonian Railway opened the Glasgow Central Line in 1896. The Glasgow District Subway, which is separate from the main lines, was also opened in 1896. The Central Line was closed in 1964, but reopened as the Argyle Line in 1979.
The Tyne and Wear Metro was opened in 1980. It took over existing surface lines and has two sections of tunnel under the city, with stations at Central, Monument, Haymarket and St James's Park.
Found in the Daily Mail's "Answer to Correspondents" column, Tuesday, 30th October, 2018, contributed by Roderick Moore of Liverpool.