eli5 Why do trains with steel wheels have traction on steel rails, but if your brakes on your car are used up and become steel on steel they dont work anymore?
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Friction force = normal force x coefficient of friction
The coefficient of friction is how “rough” the two surfaces are. For steel-on-steel this is very low.
The normal force is how much weight is pressing directly on the surface. For a train this value is immense.
Trains make up for very poor friction by weighing hundreds of tons. That minuscule friction between two steel surfaces on a tiny contact patch is magnified by 40,000 tons and becomes enough to stop a train.
Of course the momentum of the train is also 40,000 tons, so they can’t exactly start and stop on a dime either. They do have relatively poor traction compared to a road vehicle and can’t go up or down steep grades.
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