What are two ways friction can be reduced and what is one way friction can slow down a roller coaster?

I'm assuming that those are two separate questions.

First, you can reduce friction by either changing the temperature, considering the way the subjects react, or you can check the lubricant characteristics, considering their material and / or texture.

Secondly, a roller coaster may slow down using friction by joining two different materials. It may be metal gripping onto metal, or a bar preventing the roller coaster to continue moving. This may also be how the roller coaster slows down before entering a new action / scene.