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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Trains come to a rolling stop. Their wheels do not block and slide on the rails in normal use.
Only in emergency braking can they block and slide, and this literally shaves of parts of the wheel. They have flat spots, and need to be replaced.
In a car the brake works by letting the brake block slide over the disc. The block is made soft so it wears away and can simply be replaced while the disc lasts.
The actual tires in a car also don’t slide over asphalt in normal use, and breaking full power all the time will also rapidly destroy the rubber.
Trains brakes have various kinds, but older variants used the same concept as car disc brakes. Use a block of softer than steel material, and push it against the steel wheel. Since it’s softer, the block wears instead of the wheel.
However there’s many different ways of slowing down a trains wheels, none of them are steel rubbing against steel.
Trains also weigh a ton, so even though steel on steel friction is rather low, they can still accelerate. However this low friction shows itself rapidly when a train tries to brake quickly, or go up or down an incline.
Regular trains can only go over very low inclines without starting to slip
And when they slip their wheels get destroyed like a cars brake disc when braking without a brake block.
Some ways of solving this in just wet conditions is the train carrying boxes of sand in front of their driven wheels. They just sprinkle the rail with sand to have more grip in automn/when wet/leaves on the rails.
And for going up steeper inclines they use sprocket drives and stuff like that.
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