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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Steel on Steel actually has a decent coefficient of friction – about 0.4 when sliding and 0.8 when stationary.
The issue for trains is that they have a relatively low contact area (a result of hard wheels and tracks to reduce rolling resistance) and are very heavy.
If a train full on slams the brakes and stops the wheels from rolling they’ll start to slide, and the shear force on the wheel (due to the fairly high friction) will be greater than the shear capacity of the small section of wheel material in contact with the tracks, so will shear off and result in a flat portion on the wheel. This is obviously bad, so instead trains brake slowly to avoid this type of uneven wear on the wheels. Note that in this process the wheel is always rolling, not grinding, and the brake pads for the train work in a similar way as those on a car – the energy is still dissipated through the pads, not the wheels on the tracks except in an emergency.
In a car braking system the same typically applies – most braking tries to avoid skidding (both to prevent tire wear and because once skidding you actually lose braking power).
The pads, in both systems, exist to provide a way for the braking system to wear without damaging the brake disc/wheels. Both trains and cars could have steel on steel brake pads, and the coefficient of friction for this isn’t bad, but they’d wear down very very quickly.
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