What's the difference between metro, subway and underground?

Metro-a noun, is the abbreviation of "METROpolis", which originally means "metropolis". The subway in many cities uses "metro", mainly because the name of the subway company is "metro".

For example, the subway in Washington DC is called METRO.

Subway terms. Subway. Real subway. It can be on the ground or underground, although the original meaning of the word is "underground road"

For example, the SUBWAY in new york is Subway, because no one remembers the name of the subway company.

Underground-adjective, underground. This word can only be used to describe the subway underground, the car underground and so on. , or "underground workers".

But many people use underground as a noun just because they are lazy. Over time, everyone called it this way, and it became a well-known noun.

For example, there is a subway in Philadelphia, but people don't call it a subway. They call it an underground bus.