A force called centripetal force. Please check the related link to know more.
Momentum
centripetal force
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.
Forces on roller coasters primarily include gravity, inertia, and friction. As the coaster ascends, gravitational potential energy increases, which is converted to kinetic energy as it descends, causing acceleration. Inertia keeps passengers pressed against their seats during rapid turns and drops. Additionally, friction between the coaster and the track affects speed and energy loss throughout the ride.
On a loop-de-loop roller coaster, you stay in your seat primarily due to the forces of gravity and centripetal force. As the coaster descends and accelerates, these forces work together to hold you securely against the seat, preventing you from falling out during the loop. The design of the ride, including harnesses and safety restraints, further ensures that you remain safe and in place throughout the experience.
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.