🚇 Subways and Metros: Cities Beneath the Street

What you’ll learn

1. Why Go Under at AllExplain why cities pay extraordinary sums to put railways underground — that the purchase is an exclusive right-of-way in a plane where nothing else competes.A city's surface is a fully occupied plane, and a railway needs a continuous strip that nothing may cross — which in a centre does not exist at any price. The real problem is conflict, not distance. Going underground spends capital once to buy a right-of-way that conflicts with nothing, and the Metropolitan Railway proved both the demand and the physics on 10 January 1863, when it ran steam locomotives through a tunnel.
2. Cut and Cover: Wreck the Street, Then Rebuild ItDescribe cut-and-cover construction, distinguish bottom-up from top-down, and explain why shallow stations are an advantage rather than a compromise.Cut-and-cover requires no tunnelling at all: dig a trench, build a box, put the street back. Bottom-up excavates fully in the open; top-down builds walls and roof first so the surface can be reinstated early while excavation continues underneath. Shallow stations put the platform seconds rather than minutes from the pavement, which is worth real money — and early metros stayed shallow partly because proximity to the surface helped ventilate steam. Its limit is absolute: you must destroy everything above the tunnel to build it.
3. The Bored Tube: Leave the Street AloneExplain how a TBM holds the ground while it digs, and why the resulting tunnel is circular.A TBM combines a rotating cutter head, a thrust system, muck removal and a segment erector, and advances by pushing hydraulic cylinders against the concrete rings it has already built — the tunnel is its own foundation. The hard problem is holding the face: earth pressure balance uses the excavated muck itself to maintain face pressure, while a slurry shield uses pressurised bentonite for high-water and granular ground. Tunnels are circular because ground pressure at depth becomes pure compression around a ring, which concrete resists superbly.
4. The Price of Going Deep: A Tunnel That Never CoolsExplain why the deep tube is hot — that a sealed tunnel has no outside — and why air conditioning on deep tube trains would make the network hotter.London clay was around 14 °C when the tubes were built and acted as a vast free heat sink; it has now risen to 19–26 °C and reached its thermal capacity. The heat is not from passengers (3%) but overwhelmingly from braking losses (38%), because a train that stops every ninety seconds must convert its kinetic energy into heat. Air conditioning cannot fix it: a cooler relocates heat rather than destroying it, and in a sealed tube there is nowhere to relocate it to — so the trains get cooler and the tunnels get hotter.
5. Power at Track LevelExplain why metros take power from a third rail — that tunnel cross-section economics decide it — and what that choice costs operationally.For the same vehicle size, third rail needs a smaller vertical structure gauge than overhead line, allowing smaller tunnel cross sections and real construction savings on every metre bored. The costs are a ~600–750 V DC ceiling (above ~1500 V is unsafe at ground level), substations roughly every 2 km, gapping at junctions, icing on top-contact rails, and a practical speed limit around 161 km/h. For a train that stops every kilometre, that speed ceiling costs nothing — so the weakness is free and the strength pays everywhere.
6. Breathing: The Grates, the Fans, and the Piston EffectExplain how a metro tunnel is ventilated — the piston effect and shafts — and why ventilation matters far more for smoke control than for cooling.A tube train nearly fills its tunnel, so it acts as a piston: it pushes air ahead of it and drags air in behind, and heat is extracted through ventilation shafts driven by this piston effect or by fans. Thermally, though, this whole visible apparatus removes only about 10% of tunnel heat against the 79% absorbed by the walls. Its indispensable job is directional smoke control — in a fire, the evacuation route and the chimney are the same object, a lesson driven home by the 1987 King's Cross fire.
7. Dwell Time: The Real CeilingDerive metro capacity from headway and dwell time, and explain why dwell — not tunnelling or braking — is the binding constraint on a metro line.Capacity is n_pas = P × (3600 / T_min): train capacity times trains per hour. P is fixed in concrete by the tunnel bore, so every real capacity gain attacks T_min — and on a metro, T_min is governed less by braking distance than by how long a train occupies a platform without moving. Dwell time therefore sets capacity, and it is a reinforcing loop: long dwell stretches headway, a longer gap accumulates more passengers, and more passengers lengthen the next dwell. Airport lines prove the rule — same trains and tunnels, but luggage lengthens boarding and capacity falls.
8. More People Than the AirlinesGrasp the scale of what metros actually move, and close the course's argument that the tunnel is the easy part.In 2024 the Shanghai Metro carried 3,773.8 million passenger journeys, Beijing 3,621.3 million, Guangzhou 3,255.0 million, Seoul 2,710.4 million and Tokyo Metro 2,496.8 million. Against an estimated 5 billion passengers for all the world's airlines in 2024, one city's metro approaches the global airline industry as a count of times a person was moved — though the units differ and the claim must be made carefully. The course closes where it began: the bore is solved, but heat and dwell, the two problems the tunnel handed back, are the real subject.

Questions this course answers

Grounded in trusted sources

• Wikipedia — London Underground: https://en.wikipedia.org/wiki/London_Underground
• Wikipedia — London Underground cooling: https://en.wikipedia.org/wiki/London_Underground_cooling
• Wikipedia — Cut-and-cover: https://en.wikipedia.org/wiki/Cut-and-cover
• Wikipedia — Tunnel boring machine: https://en.wikipedia.org/wiki/Tunnel_boring_machine
• Wikipedia — Third rail: https://en.wikipedia.org/wiki/Third_rail
• Wikipedia — Headway: https://en.wikipedia.org/wiki/Headway
• Wikipedia — Dwell time (transportation): https://en.wikipedia.org/wiki/Dwell_time_(transportation)
• Wikipedia — List of metro systems: https://en.wikipedia.org/wiki/List_of_metro_systems

Every Wunder lesson is built from real, reputable sources — never invented.

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