Yes, there is more surface area on a coaster train than a human being. But, coaster trains also are aerodynamically designed, unlike humans.
But, there is also more inertia. What this means is that although the accelleration of gravity is constant, regardless of an object's mass, objects with more mass can reach higher speeds during freefall than objects with lesser mass and similar forces of friction.
Remember, the definition of inertia is "A body at rest tends to remain at rest unless acted upon by some force. A body in motion tends to remain in motion unless acted upon by some force."
In this case, the forces are gravity and friction.
And, yes I do know there is friction between the rails and wheels, hell, even within the wheels themselves, there is still a lot of inertia. This is one of the areas Arrow missed on the Steel Phantom. When first built, there was too much speed going into the loops. This is because they didn't give the train enough 'uphill' to lose its momentum (inertia).
As a related side note, inertia is what causes trains to be able to go through loops. There really is no such thing as centrifugal or centripetal force. These are terms to describe inertia at work. It only feels like you are being forced down into your seat, when in reality, the seat is being forced up into you. Because of your inertia, you tend to move in a straight line. The train forces you to move in another direction.
I can hear all of you snoring now......