# How do train wheels not skid or lose traction?

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Basically the title. Considering that train wheels are made of metals and run on metal tracks, how do they prevent wheelslip or loss of traction especially when starting with heavy loads or when going uphill? I’ve heard of sandboxes which can be used to provide more grip by spraying sand onto the track but I’m not sure if they’re still prevalent.

Thanks!

In: 72

Trains are very very very heavy. Friction is determined by two things: the ‘slipperiness’ (coefficient of friction) between two things and the force of of the things pushing together (normal perpendicular force) for example a heavy train pushing down on the track. So you are right, metal on metal is a bit slippery, but the train is super heavy.

newer train wheels are angled like trapezoidal. Keeps train centered on track especially when turning and differential speed happens. Pretty cool actually

The friction coefficient of steel on steel is quite good.

https://www.engineersedge.com/coeffients_of_friction.htm

I suppose the idea that metal on metal is slippery comes from having seen some lubricated linkage. Remove the lube and it grinds bad.

Or maybe from ball bearings. Which are lubricated too. But regardless. Ball bearings have no friction. All contacts are rolling contacts.

You’re correct that the friction between the wheel and the rail is much less than that of a rubber tire on a road. Because of that, the acceleration of trains is usually much slower than that of a passenger car. Many modern passenger trains are what’s called electric multiple units, where many of the axles along the train are powered. This gives the train a higher portion of the total mass that rests on powered axles, usually allowing them to accelerate faster than locomotive-hauled trains.

Sometimes the wheels do slip and sand is still used to mitigate this during certain less favorible conditions. When the wheels slip during braking it can cause flat spots on the wheels which you can hear as a train passes. These have to be removed by machining a layer of metal from the wheels.

Leaves are particularly bad for traction on railways, so much so that some passenger railways even adjust their timetables to add extra time during the fall to account for the slower acceleration and braking.

They do. Any British computer can tell you that “leaves on the line” Is a common reason for trains being late. The leaves get crushed into a paste, which reduces friction and the wheels slip (on acceleration, braking, or climbing)