If it's jumping out on the outside of a curve, the chances are excellent that there's a height disparity across the rails and that there's a dip on the outside rail. It will probably do this in slow motion, but not necessarily. Slow it down, and put strong light on that spot. Watch for the flange to clear and pop out and where it happens. But wait, there's more! While there may be a low spot there, it could also be that the entire frame is being lifted by what is happening back on the second engine's drivers, where the rest of the weight is being supported (model trucks support no weight, unlike the prototype). Maybe that part of the curve has an outer rail a little high. As the rear engine's lead driver encounters the high spot, it lifts the frame some and lightens the load bearing on the outside tires in the lead engine.
Find a bubble level, one of the dot-type that you can set on the rails, and slide it along the entire curve slowly. Watch for rolling evinced by the bubble moving sideways. You'll soon know where the elevations change across from each rail. Once you have your rails fixed, especially with no dips, your Challenger should move through that curve nicely and reliably.
