There is usually some play between the upstops and the bottom of the rail. Magnum's may be a little bit more than normal, but a little tolerance is needed to prevent binding. B&M helps themselves out w/ their spring system, but there are some other design changes that allow for that. ST, for instance, as a wooden has some play. Indeed, at times it crests its fourth hill with no wheels on a car touching the track at any point. All 8 wheels [2 road, 2 guide, 4 upstops] are airborn and floating in that tolerance.
Wheel tolerance is part, but remember, Arrow's loopers use trailered trains too. This includes Steel Phantom, which is notoriously painful. Although to be fair, this is more a restraint problem than a tracking one.
I'm particularly impressed w/ Setpoint's truss system for the Seizmic, which is a box constructed from triangles, essentially.