The Mountain Coaster is an amusement park attraction introduced in Silent Hill 3. Heather Mason explores it twice, once in the opening nightmare sequence, and once when she visits Lakeside Amusement Park in reality.
In the opening nightmare, she must find her way to the roller coaster and then walk along the tracks. A roller coaster car comes along, knocking her off the tracks, probably to her death. After this scene, she wakes up in Happy Burger.
The second time, when she visits the park while awake, she needs to find the roller coaster key in the Sweet Factory Store store to avoid the death that occurred in her dream. The key unlocks the roller coaster's control room so she can turn off the ride, although this ends up being little help.