Kid Asked

Why does a heavy train not crush the rails?

Tap the yellow button to begin.

For the grown-up

The story shows the most common kind of track: steel rails on wooden ties on crushed stone. Here is what is going on underneath, where the story stops being true, what to ask, and what to look for from a safe distance.

How a track carries a train

  1. Wheel. A big freight engine weighs about 190 tonnes, as much as a hundred cars, and stands on twelve steel wheels. That is about 16 tonnes on each wheel. A wheel touches the rail on a patch of about one square centimetre, the size of a fingernail.
  2. Rail. The rail is a long, stiff, springy beam. It dips a few millimetres under the wheel, and that hands the load to several ties at once: about 40% to the tie under the wheel and a little under a quarter to each of its neighbours.
  3. Ties. The story calls them blocks. Each one is far wider than the rail, so it presses on the stones over a bigger area. Ties also hold the two rails the right distance apart.
  4. Stones. The crushed rock is called ballast. The pieces are jagged, about 5 cm across, so they lock together and pass the load down and outward. Rain drains straight through them.
  5. Ground. The squeeze starts at nearly 1 gigapascal under the wheel, about 10,000 times the pressure of the air. At the soil it is about 40 kilopascals. By our own sum, that is roughly what a grown-up puts on the floor standing on one foot.

Why steel

Rail steel is iron with a little carbon in it. A common European grade only breaks at about 900 megapascals: a bar no thicker than a fingernail is wide could hang nine tonnes.

It is also elastic. It bends under a load and comes back to where it was. A US engineering guideline is that a rail should not dip more than a quarter of an inch (about 6 mm) under a wheel.

The clay and the cookie stand in for real history. In 1804 the cast-iron plates of a Welsh tramroad broke under Richard Trevithick’s five-tonne engine: too brittle. Later, iron rails at one busy spot in Derby station had to be renewed every three to six months: they wore out too fast. A steel rail laid there in 1857 lasted 16 years.

One metre of main-line rail weighs 50 to 70 kg, nearly as much as an adult.

Where the story stops being true

“It does not squash”
Not quite. On the tiny patch the pressure is higher than the point at which steel starts to give, so a thin skin of metal does flow. Over years the rail wears down, small cracks start a few millimetres below the surface, and bits can flake off. Railways measure rails and replace them once 10 to 25 mm has worn away. Rails do sometimes break: several hundred a year are recorded in Britain.
The springy page
The bend is drawn many times too big. A real rail moves by millimetres.
The push picture
It is tidier than life. The 40% is a typical figure and changes with how far apart the ties are and how firm the bed is.
One block in the mud
A made-up test, though the idea is real. Track does sink where the ground is soft or wet, and crews use a machine called a tamper to pack the stones back under the ties.
Not every track
In North America about nine ties in ten are wood. Many other countries use concrete, and some lines have no stones at all, only a concrete slab. In Britain a tie is called a sleeper.

Questions to ask

  • Before you start: “Why do you think the rails don’t break?” Let the guess stand, then find out together.
  • “What if the rails were made of wood? Or glass?”
  • “Why do your boots sink in mud but not on the sidewalk?”
  • “Where else do we spread a push?” Snowshoes, skis, a board laid on soft sand, lying flat on thin ice instead of standing.

Try it

  • One foot or a board. In a sandbox or in snow, stand on one foot and look at the hole. Then lay a board or a tray down and stand on that.
  • Coin and book. Press the edge of a coin into play dough. Then lay a book on the dough and press the coin on the book.
  • Springy steel. Hold a metal ruler flat on a table with half of it over the edge. Push the end down, then let go.
  • Toy track. Lay a piece of toy track on a pillow and push a train along it. Then put a tray under the track.
  • Spot the parts. From a station platform, a bridge or behind a crossing gate: is the top of the rail shiny? Can you see the ties and the stones? There are about two ties for every metre of track.

Stay back from real tracks

Look, but never from the track. Tracks are private property, and being on or close to them is dangerous and illegal. A train can take a mile or more to stop. Expect a train on any track, at any time, from either direction.

Where this comes from

The voice is a computer voice. With the sound off, the words stay on the page for you to read aloud.

What did your kid ask this week?

Send it in their own words. We read every one, and we paint the ones we can answer truthfully.

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