Reply to "Learning the hard way"

Instead of redoing the whole thing, just change the grade of the last 1/4 of the spiral to be something less, and that could help the locomotive.  Also, have you tried it with the coaches behind the locomotive?  Yes, it is still downhill, but they provide friction and will change the vector of the force on the locomotive as it enters the last curve, and it may be enough to keep it on the rails.