How can trains move on rails? if the wheels are smooth and the rails are also smooth, how can it be enough friction for it to move?
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Just the locomotive (the engine car) weighs close to ½ million pounds. Even if the wheels and rails were polished mirror smooth, there’s still friction
Just the locomotive (the engine car) weighs close to ½ million pounds. Even if the wheels and rails were polished mirror smooth, there’s still friction
Because the force of friction is equal to the coefficient of friction multiplied by sin of the angle perpendicular to the surface, times the weight of the object, and trains are HEAVY. A single car weighing 120 tons is not uncommon.
Because the force of friction is equal to the coefficient of friction multiplied by sin of the angle perpendicular to the surface, times the weight of the object, and trains are HEAVY. A single car weighing 120 tons is not uncommon.
You can place a penny on the track and it will be literally [flattened](https://qph.cf2.quoracdn.net/main-qimg-a0bac7993cbd3ff14b8deb69aebb47ce-lq) by the train. That train is constantly trying to flatten that rail too – that’s all the friction you need.
You can place a penny on the track and it will be literally [flattened](https://qph.cf2.quoracdn.net/main-qimg-a0bac7993cbd3ff14b8deb69aebb47ce-lq) by the train. That train is constantly trying to flatten that rail too – that’s all the friction you need.
This is the equation for friction:
>f = u N
>
>f = The force of friction
>
>u = The coefficent of friction
>
>N = the Normal force (for this conversation: how heavy it is)
You’re right, round metal wheels on flat metal bars has a pretty low coefficent of friction. However trains are fucking giant and heavy. That means they both have a high Normal force, and also don’t need that much friction since they accelerate/decelerate slowly due to high inertia.
This is the equation for friction:
>f = u N
>
>f = The force of friction
>
>u = The coefficent of friction
>
>N = the Normal force (for this conversation: how heavy it is)
You’re right, round metal wheels on flat metal bars has a pretty low coefficent of friction. However trains are fucking giant and heavy. That means they both have a high Normal force, and also don’t need that much friction since they accelerate/decelerate slowly due to high inertia.
UK passenger train driver here. This has been answered better than I could but I’ll weigh in on some real world stuff.
We do have sand we can put onto the track in front of the wheels at the push of a button but it is only really needed in autumn. Crushed leaves on the line create some kind of residue that makes the rail head really slick. With modern trains we have wheel slip protection (WSP) systems which works kinda like abs in a car so it’s not as frightening as it used to be when the wheels just lock and everything goes silent when you try to brake. Getting moving is usually harder as the wheels slip and the engine cuts out briefly. Sand helps a lot here.
Light rain in autumn when there’s residue on the tracks really compounds the problem. I’ve had a train stuck on a hill on a wet autumn day that just wouldn’t go forwards. It happens a more with heavy freight trains.
Some of our newer trains really have a lot of power and even on a dry summer day you can go straight into full power and the wheels will just spin so we start in low power and notch up.
We mostly drive multiple unit (MU) trains rather than locos which means each carriage has it’s own engine and brakes so I guess that makes the situation better (until you lose an engine).
UK passenger train driver here. This has been answered better than I could but I’ll weigh in on some real world stuff.
We do have sand we can put onto the track in front of the wheels at the push of a button but it is only really needed in autumn. Crushed leaves on the line create some kind of residue that makes the rail head really slick. With modern trains we have wheel slip protection (WSP) systems which works kinda like abs in a car so it’s not as frightening as it used to be when the wheels just lock and everything goes silent when you try to brake. Getting moving is usually harder as the wheels slip and the engine cuts out briefly. Sand helps a lot here.
Light rain in autumn when there’s residue on the tracks really compounds the problem. I’ve had a train stuck on a hill on a wet autumn day that just wouldn’t go forwards. It happens a more with heavy freight trains.
Some of our newer trains really have a lot of power and even on a dry summer day you can go straight into full power and the wheels will just spin so we start in low power and notch up.
We mostly drive multiple unit (MU) trains rather than locos which means each carriage has it’s own engine and brakes so I guess that makes the situation better (until you lose an engine).
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