rail · self-propelledgrade-separated

Metro / Subway

the highest-capacity mover on the map

A metro is what happens when you decide a city's busiest rail line will never wait for anything — not a car, not a pedestrian, not another train. Give it its own tunnels and viaducts, seal it off completely, and it turns into the densest people-mover any city owns.

How it works

A metro — also called a subway, underground, or simply rapid transit — is high-capacity urban heavy rail running on an exclusive right-of-way. Steel-wheeled trains ride steel rails, drawing electric power from a third rail beside the track or from an overhead wire. Trains are long (often 6–10 cars), run at short headways, and stop only at stations. Everything that would slow them down — cross traffic, level crossings, shared street space — has simply been engineered out.

Grade separation is the whole trick

The defining feature is full grade separation: the line has no level crossings anywhere. It runs in bored tunnels, in cut-and-cover boxes under the street, or up on elevated viaducts, but it never shares a plane with roads or footpaths. That single decision is what unlocks everything else — because nothing can stray onto the track, trains run fast, frequent, and automatic-friendly without the safety compromises street running forces.

Capacity follows directly from separation. A typical line runs about 36,000 passengers per hour per direction, and the busiest East Asian trunk lines push into the range of 75,000–85,000 per hour — throughput no bus corridor or street tram can approach.

This is the sharp line between a metro and a tram or light rail, which trades separation for cheaper street-level running: trams mix with traffic, stop at signals, and move far fewer people per hour. Same steel wheels, opposite philosophy.

Steel wheels, rubber tyres, and no driver

Most metros run steel-on-steel, but a notable family rolls on rubber tyres instead. Paris pioneered rubber-tyred metro lines, and Montreal and Mexico City built entire systems around the technology. Rubber tyres grip better on grades and start and stop more quietly, at the cost of higher rolling resistance and more heat.

Third rail vs overhead — a trackside conductor rail for compact tunnels, or catenary where clearances allow.
Steel wheel vs rubber tyre — the default versus the Paris / Montreal / Mexico City approach.
Driverless operation — full grade separation means no one can wander onto the track, so many modern lines run fully automated, with no driver aboard and trains timed to the second.

From 1863 to the world's busiest

The idea is old. The London Underground opened in 1863 as the world's first rapid transit system — the partially underground Metropolitan Railway, hauled at first by steam locomotives before electrification made deep tunnels livable. From that single line the pattern spread worldwide. Today the busiest systems by ridership are in East Asia — the Shanghai Metro, the Tokyo subway, and the Seoul Metro — each carrying billions of trips a year. It remains the highest-capacity mover on this map, and the reason is always the same: it answers to no crossing.