What is the full form of metro?

The full form of metro depend on the context as it can stand for Metropolitan or it can refer to a rapid transit system (subway), also known as a metro. 

Metro is often used as a shortened form of metropolitan, referring to a large city and its surrounding area.

In the context of transportation, a metro is referred to as a rapid transit system, which is usually an underground railway (subway).

Rapid Transit is a general term for a high-capacity urban rail system, of which metro trains are a common example.

Mass Rapid Transit (MRT) is used to describe the same type of high-capacity public transport system found in urban areas.

Heavy Rail is another term for the type of rail system that is often used for metro trains.