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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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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This is a challenge for trains, there’s only a little friction and a tiny surface area – this why trains have quite low gradient limits.

However, because there’s so little rolling resistance, once a train gets going on a flat track it takes far less energy than a truck or rubber wheeled vehicle would take to keep moving. So you save a huge amount of energy (and thus fuel).

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

Trains are heavy, monstrously heavy, so that weight helps with the lack of friction somewhat.

The engines also have grit blowers (basically sand) in front of the driving wheels to increase friction and thus traction so the machine can get moving.

Once a train gets moving, it takes little energy to keep them moving, so there are quite a few benefits to using steel on steel to move goods around.

There are other methods as well, especially for dealing with inclines and declines but that is getting deep into the process.

Because friction is only very low, but it is not zero. We actually want low friction to save energy.

Trams and trains generally carry sand to put on the tracks to increase friction temporarily (e.g. steep inclines in ice and snow, or emergency breaking).

Steel on steel actually creates a lot more friction than people in this comment section are making it out to be….