The roller coaster (some people say the simple form coaster) is a kind of theme park ride. A roller coaster is usually a small train. It has a number of carriages that travel on a track. This track has hills that go up and valleys that go down. Sometimes it has other features that will turn the train upside-down. The track can be made from wood or steel. Some roller coasters are made for children, so they do not travel very fast. Other roller coasters are made very tall and they travel very fast. Riders are generally held in place with a bar, a belt, or something similar.