A force called centripetal force. Please check the related link to know more.
centripetal force
Momentum
If in roller coaster designer you get the coaster to go underground, then that's the problem. If you load that coaster you can't put a part of it underground.
inertia
Restraints and centrifugal force.
Kinetic energy
The wheels are on top, between and below the tracks.
There is a few principals that are looked at when building a roller coaster. Some principals that are looked at are what keep coaster cars flying around on their tracks, the tolls that keeps everything running and the forces that make the ride fun.
inertia. because inertia keeps things going in a straight line, and that kind of throws it forward, but the direction of "foward" keeps changing as the car or roller coaster turns.
Previous Answer => "Because of the law of inertia."Improved Answer => Well, yes I suppose, but it is not necessary based on that fact.Since roller coasters have a special three-wheel design that keeps coaster trains on the track NO MATTER WHAT, banking doesn't "keep the ride on the tracks". If anything, banking keeps the coaster safe (in terms of G's) and comfortable to actually be enjoyed versus being painful. A coaster and it's riders will follow the track no matter what, since again of the three-wheel design patented by John Miller in the 1920's AND the restraints on rides that keep people in. It is the G's that concern designers, not if the train will fall off.
I suppose you're talking about the loop-dee-loops? It's G-Force. The bar helps also. :)
When the roller coaster starts to stop your body wants to keep moving, that is inertia.