A subway is a railroad system that moves people quickly through a city, usually underground. Instead of trains running on tracks at street level, subway trains travel through tunnels beneath the roads and buildings above. Cities like New York, London, Paris, and Tokyo use subways to transport millions of passengers every day.

Subways solve a problem that crowded cities face: how to move huge numbers of people without clogging the streets with traffic. When you ride a subway, you descend a staircase or escalator into a station below ground, wait on a platform, and board a train that whooshes through dark tunnels, stopping at stations along its route. Above your head, regular life continues as cars drive, people walk, and buildings stand, all while you're traveling in a completely separate world underneath.

Some cities call their subway systems by other names: London calls theirs “the Tube” or “the Underground,” and in some places you'll hear “metro” instead. But they all mean nearly the same thing: a fast, efficient train system connecting neighborhoods and helping people get where they need to go without sitting in traffic.