'Metro' and 'subway' are used synonymously throughout Metrobits. Metro systems can move people within cities more efficiently than any other means of transport. A striking feature is that they often run in tunnels and stop at underground stations. Hidden from view, like the arteries of a living organism, they keep the big cities alive. The dense skyscraper districts of modern cities couldn't exist without metros.
A popular definition from [urbanrail.net] defines a metro as
an urban electric mass rail transport system, i.e. it is primarily used to move within the city,
totally independent from other traffic, rail or street traffic,
providing high-frequency service (maximum interval approximately 10 minutes during normal daytime service).
However, definitions of what a metro is can vary according to who you ask. Furthermore, the differences between heavy rail, metros, and trams are increasingly fading since the middle of the 20th century, as many metros or light rails have gradually evolved from trams or regional rail lines, and new hybrid trains are able to interconnect different systems. This makes it sometimes difficult to decide whether a rail line should be counted as a metro or not.
This page: http://mic-ro.com/metro/metrodefinition.html
