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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The friction is low but so is the rolling resistance, that is why trains use steel wheels on steel tracks.
Locomotives are heavy so even if the friction is low the max friction force is directly proportional to the weight.
Low in this case is higher than you expect.
If you look at [https://www.engineeringtoolbox.com/friction-coefficients-d_778.html](https://www.engineeringtoolbox.com/friction-coefficients-d_778.html) the steel-steel friction is 0.5-0.8 for a dry and clean surface in a lab but is typically 0.35 to 0.5 in reality. Car tires and asphalt is 0.7.
In extreme conditions, it can be as low as 0.05 for railroads, crushed leaves that leave an oil film are an example, which is comparable to tires on the ice at 0.1. That is a tire with just rubber and no studs or chains.
Trans often have a sand system that adds some sand in front of the wheel if used in extreme condition.
So in dry condition railroad have friction around half that of a car
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