system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are train tracks smooth?

If friction is the force that pushes a vehicle forward, wouldn't you want as much friction as possible like a car tire? Is the train so heavy that a low coefficient of friction is compensated for?
gpt
Friction is also one of the main forces slowing you *down* (along with air resistance). Remember that objects in motion will stay in motion until another force acts on them. That means that once the train gets going, you don't *need* friction to keep going and all it does is act against you.

With cars, you need friction because you need to stop often and quickly, and you need to turn quickly and sharply. Trains don't have those limitations. The tracks control their direction and only take long, gentle turns. The tracks stay open and empty through careful coordination, so trains rarely need to stop until they get close to their destination. 

The low friction means that trains do take a very long time to start moving, and to stop. The fact that they're massive also makes them take forever anyway. The low friction also means that they don't need to use a lot of energy to keep going once they get going. That makes trains *very* efficient.