Science Puzzle
How Does a Train Turn a Corner?
A train's wheels are fixed in pairs to a solid steel axle, so both wheels on an axle always turn together at exactly the same rate. On a bend, though, the outer rail is longer than the inner one. The outer wheel has further to go than the inner wheel in the same time.
A car has a box of gears called a differential that lets its wheels turn at different speeds. A train has nothing like that. So how does it get round a bend without its wheels skidding?
The Answer
The wheels are slightly cone-shaped, so each axle steers itself round the bend.
The rolling surface of a train wheel isn't a flat cylinder. It tapers gently: it's biggest next to the flange on the inside and a little smaller towards the outer edge. On a bend, the train swings outwards slightly, so the whole axle shifts towards the outer rail. Now the outer wheel rolls on its bigger part and the inner wheel on its smaller part. A bigger circle covers more ground in one turn, so the outer wheel travels further than the inner one even though both turn at the same rate. The pair curves round, just as a paper cup rolled across a table runs round in a circle.
The same trick keeps a train centred on straight track. If an axle drifts to the left, the left wheel rolls on its bigger part and the right wheel on its smaller part, which steers the axle back to the right. It overshoots a little and comes back again, so the wheels gently weave from side to side as the train goes along.
The flanges are a safety net. On ordinary bends they hardly touch the rail at all; if they did the steering, they'd grind themselves away. On very tight bends the cone isn't enough and the flange does rub against the rail. The screech you hear as a train creeps round a sharp curve comes from exactly this.
The principle: Coned wheels. On a fixed axle, tapered wheels can each roll on a different size of circle, so on a bend the outer wheel covers more ground and the axle steers itself round.