London has a famous underground railway system which we locals call the 'Tube'. This is because many of the tunnels are a round tube shape. Although people call it the 'Underground' or 'Tube', half of the stations are actually above the ground.
In America the first metro was built in New York, and it was called SUBWAY because it was built mostly under the streets (sub= under / way= road), since Manhattan street plan is quite a regular grill and it was easy to do so. But most everywhere else they usually call it simply the TRAIN.
Rapid transit or mass rapid transit (MRT), also known as heavy rail or metro, is a type of high-capacity public transport that is generally built in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be called a subway, tube, or underground.
The British call cookies biscuits. They occasionally use the word cookie in the context of using Americanisms like he got caught with his hand in the cookie jar, or that's the way the cookie crumbles.
In terms of strict dictionary definitions, both terms imply underground trains. Subway is a generic term, but metro is more localized to certain cities. Subway is generally used for underground train systems and metro for above ground.
There are around 20 trains per day from London to Paris. The comfort level of Eurostar train, affordable prices, fast speed and luxurious service makes it a winner over all other travel option like ferries, buses and flights.
THE world's first metro, now the world's oldest system, is the London Underground in England, which is more commonly known as the Tube, which was opened in 1863.