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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The trains weigh a lot ^((citation needed)), so they generate a lot of friction force.
You could use more force on your worn out brakes and they would work, or you could wait a long time. But the brake material means you can use less force and work much faster. The downside is you have to replace them more often. (note also that the grip of the tires is very important for braking too). For a train this would be both cost prohibitive and unnecessary since the inherent weight of the train already provides the necessary force to create friction for movement.
It’s also worth noting that trains take forever to stop and they can’t go up much of an incline, so the steel on steel isn’t giving them the best performance possible, but for the purposes they are designed for it’s a good balance of cost and performance. Even if we could make them stop faster, that would almost certainly result in catastrophic failure including derailment. You can’t just stop 1 million tons of cargo on a dime. We expect different performance characteristics for a car (for example we want it to stop in a matter of feet, not miles) and thus use materials that meet that performance. For example, we use rubber tires which provide better grip but which can’t carry as much weight as a train.
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