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.
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).
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.
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
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….
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).
The simple answer is that you need a lot less force to move than you probably think. A strong person can move a train carriage by hand if it is on perfectly flat ground – all you have to is overcome the rolling resistance and let it accelerate very slowly.
In order to get something on wheels moving, you need to overcome the rolling resistance (how hard is it to make the wheels turn), the angle of the slope it’s on and any bumps it has to climb over.
Trains basically work by minimising all 3 of these factors. The tracks are very smooth so basically no bumps, the steel wheels running on greased bearings have very little rolling resistance and tracks are made as flat as possible (freight trains aim for less than 1.5% slope, or 1.5cm of height change per 1m of distance)
Once you’ve overcome the factors preventing the train from moving, it’s just mass x acceleration and trains generally don’t accelerate very quickly so they can move a lot of mass with a relatively low force
Edit: unit error…
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