Cones, not lips
Why Do Train Wheels Never Fall Off?
Train wheels are cones locked to a shared axle, so any sideways drift steers itself back.
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Step 1 of 5
The lip is the backup, not the trick.
Everyone assumes the inner lip holds the train on the rails, but on a well-behaved train it hardly ever makes contact at all.
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Step 2 of 5
Each wheel is slightly cone-shaped.
The rolling surface is not flat. It is a shallow cone, wider in diameter on the inner side and narrower towards the outside.
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Step 3 of 5
Both wheels are fixed to one axle.
Unlike a car, a train's wheels cannot turn independently. They are keyed to a solid bar, so both must always rotate at exactly the same rate.
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Step 4 of 5
Drift sideways and one wheel rolls further.
Slide left and the left wheel now touches the rail at a bigger diameter. Turning together, it covers more ground and steers the axle back.
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Step 5 of 5
Curves are handled the same way.
Leaning slightly outward in a bend puts the outer wheel on a bigger diameter, so it naturally rolls the extra distance a curve demands.
A train wheel's flange is only about 25 millimetres deep, and on well-maintained track it rarely touches the rail at all.
The short version
Train wheels stay on because they are cones locked to a shared axle, so any sideways drift makes them roll themselves back to centre.
Try it yourself
Tape two paper cups rim to rim and roll them down two parallel rulers. The cone shape keeps re-centring them with no lip at all.
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