A railway looks like two iron bars laid on stones. It is really a layered structure, designed to carry trains of thousands of tonnes to the millimetre, through rain, frost and heatwaves.
The layers, from bottom to top
- The formation: the levelled, compacted, drained ground. A railway starts with earthworks: embankments, cuttings, ditches.
- The sub-ballast: a layer of compacted gravel that spreads the loads and stops the soil working its way up.
- The ballast: crushed stone, hard and angular, about 25 to 50 mm, some thirty centimetres deep. Its angles lock together and hold the sleepers; its gaps let water through. Round pebbles wouldn't hold anything.
- The sleepers (ties in North America): concrete today, creosote-treated wood on older lines, sometimes steel. About 1,666 sleepers per kilometre, 60 cm apart.
- The fastenings: elastic steel clips that press the rail onto the sleeper, with a rubber pad between them. On wood, large screws called coach screws.
- The rails: very hard steel. Main lines use rails of 60 kg per metre.
Gauge
The distance between the inner faces of the two rails:
| Gauge | Name | Where |
|---|---|---|
| 1,435 mm | Standard gauge | Western Europe, North America, China, high-speed lines |
| 1,520 mm | Russian broad gauge | Russia, Ukraine, the Baltic states, Finland (1,524) |
| 1,668 mm | Iberian gauge | Spain and Portugal (except high speed) |
| 1,000 mm | Metre gauge | Mountain lines, Africa, Southeast Asia |
At borders where the gauge changes, bogies are swapped, loads are transferred, or variable-gauge wheelsets are used. See The bogie.
Continuous welded rail
Modern rails are no longer laid in lengths with joints: they are welded end to end over kilometres, by electric welding in the factory and thermite welding on site (a reaction of aluminium powder and iron oxide that produces liquid steel, poured into a mould between the two rails).
A one-kilometre rail would like to grow by almost 50 cm between winter and summer. It's prevented from doing so: it's laid under tension, at a chosen temperature, and the ballast and fastenings hold it. In a heatwave, if the track is poorly maintained, it can suddenly kink sideways: that's buckling, one of the reasons for summer speed restrictions.
What you find around it
- Cant: in curves, the outer rail is raised, by up to 16 to 18 cm, to tilt the train like a banked bend.
- Switches (points): two movable blades that steer the wheels, and a frog where the rails cross. Check rails hold the wheelset while it crosses the gap at the frog. The blades are moved by a motor and locked.
- Signalling: the line is divided into blocks; electrical circuits in the rails detect a train by its axles, which short the two rails together.
- Electrification: in France, 1,500 volts DC on part of the older network, 25,000 volts AC on newer and high-speed lines, through overhead catenary.
Maintenance
Tamping machines lift the track, pack the ballast under each sleeper with vibrating tines, and bring the track back to level to the millimetre. Measuring trains run over the network to detect geometry faults and cracks in the rails.
What railway workers know
- A train can't stop. A freight train at 100 km/h (60 mph) needs about a kilometre to stop; a TGV at 300 km/h, more than three kilometres. By the time the driver sees you, it's already too late.
- The catenary kills without contact. At 25,000 volts, an arc can strike without contact, simply because someone comes too close. Every year, young people die climbing onto parked wagons. Never climb onto a wagon under overhead wires, even if the train has been standing for days.
- Rails are slippery, especially wet or icy: step over them, don't walk on them.
- A piece of rail is the best improvised anvil there is: hard, heavy steel with a flat top. Amateur blacksmiths all over the world start this way. See Le tranchant.
- An abandoned line is still a route: gentle gradients, bridges, tunnels, and a line that always leads to a station, so to a town. But a line that looks abandoned isn't always: listen, look at the rails (shiny on top means they're still in use).