In the dark underground, how does a metro find its way?

Down the stairs, through the gates, and a train slides in just as you reach the platform. Strange, isn’t it? Deep in a dark tunnel, the metro runs fast and perfectly on time. Its talent hides in three things.

Power arrives two ways. Some cities hang a wire along the tunnel roof and press a pantograph against it; plenty of lines keep the power beside the track instead: an extra rail, the third rail, whose collector shoe slides along under the train. The current completes a circuit: in through the third rail, through the train’s equipment, and back through wheels, rails and the return path to the supply — not “used electricity” being dumped anywhere. The two running rails carry the train and double as the way back. Which one a city picks depends on tunnel space, history and cost.

The line is cut into squares. How does a train know whether the track ahead is clear? The line is divided into short stretches — blocks — like squares on a running track. Every block a train’s full length overlaps counts as occupied, and the blocks tell the signalling system “there is a train here”; seeing occupation ahead, the following train brakes early, keeps its braking distance, and stops to wait when it must. Signalling, braking and dispatching keep the line safe together — the squares are a picture that helps us understand spacing, not the whole reason collisions cannot happen. This diagram is an illustration, not a real safety-system simulation.

Stopping exactly right. Every time, the train’s doors line up with the platform screens like puzzle pieces. The onboard computer counts the distance all the way, easing off early so the train glides to the bottom of its “slide” — to within a few dozen centimetres. Doors are enabled after the train stops and its position and opening side have been checked.

At the terminus, how does the train come back? One common arrangement hides a turnback track behind the station: the train pulls in, stops, and the driver walks to the cab at the other end — head becomes tail, and off it goes the other way. A metro’s “turnaround” is really just a change of head; other lines use loops or different turnback layouts.

Long tunnels must breathe: in normal running, big fans push fresh air in and stale air out, and the trains themselves pump the air like pistons. In a fire or other emergency, a separate smoke-extraction system starts — note: the direction of the airflow is not the escape direction. Follow the staff and the announcements, use the marked escape routes, and never enter the track. A metro line moves enormous crowds and dodges street congestion, but at peak times or during maintenance it queues and waits like anything else.

Detailed controls

1. Add trains. In the exploration, raise “trains online” from 1 to 5 and watch blocks turn red and the ones behind turn yellow — even at rush hour, everyone queues one block at a time.
2. Trim the headway. Drag the interval and watch the spacing along the line change.
3. Find the third rail. Chapter two’s inset shows the shoe pressed on the reddish rail — then think: which cities run a wire overhead instead?
4. Watch the turnback. In chapter five, the driver walks from one end to the other and the train departs facing the other way.

Guess first: do all the world’s metros run on third rails? Choose, then check — no: third rail and overhead wire are both common, city by city.

This drawing divides 30 km into fifteen 2 km blocks so they are easy to see; these are not real metro block dimensions. Real lines use both fixed-block and moving-block systems. Moving-block control continuously calculates how far a train may proceed using position, speed, braking ability and safety margins. Its safe separation need not consist of equal fixed squares.

ATO (automatic train operation) measures speed and distance continuously and follows a planned braking curve; this demo uses ±0.3 m as an alignment example. Actual tolerances and the choice of automatic re-alignment or staff intervention depend on the line. Doors and platform screens only open once they “shake hands” — that is why a misaligned train simply waits.

It carries around 750 volts of DC, sits low beside the track away from platforms, and is separated from passengers during normal travel but remains hazardous electrical equipment — in an emergency, never enter the track; the walkways run along the sides. Plenty of metros have always used overhead catenary in tunnels (often rigid) — both systems run in cities around the world, cousins of the tram’s ponytail.