Brakes?

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#2
There IS friction. Without friction, the locomotive could not apply tractive effort to start the train rolling.

The contact area between the wheel tread and the rail is small and depends on the weight of the railroad car to maintain that contact, up to a point. Despite the appearance of two very hard metal surfaces, there is some elastic deformation at the point of contact to increase that contact area. Past that point, the wheel slides which can damage the wheel(s) by rubbing flat spots--I've seen wheels that literally melted away and had to be scrapped.

Contamination of the surface by water, oil and things like wet leaves can significantly reduce friction, at times to the point the train can't get up a grade, or stop in a reasonable distance. Locomotives have a supply of sand that can be applied to the rail ahead of its driving wheels to temporarily increase friction.
 



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