system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why train wheels don’t slip on the rails, when both the wheels and the rails are made of metal?
gpt
They do.  What makes trains so energy efficient is that the friction between rail and wheel is very low.  So power has to be applied very gently to stop the wheels spinning, and braking gently otherwise wheels slide.  

In about 1835 they came up with the idea of dribbling dry sand onto the rails to create a bit of friction for starting and braking, and 187 years later they still do.