They're just different names in different places. It's the “subway” in New York, The “underground” (or colloquially, the “tube”) in London. In Washington DC, the official name is the “Metro (rail)”, but a lot of people just say “subway”. In France and Spain, the say "metro". In Argentina, you’ll hear “subte”.

Here in England, we use “underground” for the railway system (specifically in London - trains in the rest of the country run above ground so underground is not used) and we use “subway” for a pedestrian walkway/tunnel that goes under the ground.

They're the same thing but different countries call them differently. underground -> US, subway -> UK,

And metro!They all look the same to me.Trains running under ground got only one name on Chinese，"地铁".