The track exerts forces that directs the momemtum of the train though gravity would pull it down to the ground and inertia would keep it moving in a straight line. That force is then transfered to the coaster as it moves along that part of the track and to the passenger through the seat of the train. The calculated force exerted on the passenger is referred to as the "seat force."
Remembering Newton's law--an object in motion will continue in motion along a straight line, you realize that passengers going have momentum pushing them straight forward. On a level turn, passengers are moving forward as the train turns...and as a result, they move to the outer side of the car. The side of the car then exerts the force that takes them through the turn. Coaster designers realized that they could make a ride more comfortable (passengers not squishing each other) and safer by banking the curves. By banking the curve, most of the force that pushes the passengers through the curve comes from the seat rather than the side of the train.
yaaaaaa
