4-6-6-4 Challenger - AZL (I will be filling this in with some tips, I do not own one, but have run them and seen some issues, so will present those here.) One thing most people mention right off is that the front truck often does not turn. Only the rear set of drivers are powered. This is a common issue, and the "fix" is somewhat problematic, after lubricating and making the front mechanism as free as possible, all you can do is add weight to the front truck. There's precious lettle space to do this. The natural impulse is to add weight to the truck, I'll look for someone doing this successfully. David Stubbs has put a small washer between the front truck pivot point and the base frame. It will apply a little more down force to front truck. He advises to put in washer and place on the track and check to see if the front wheels of the truck lift, then remove the washer and use a thinner washer. By experimentation, he has gotten to a "a sweet spot" that keeps it rolling without wanting to derail. There is also an issue with excessive wear in the "axle pin" that supports the worm gear, that is the gear between the worm and the rest of the gearing to the truck. The hole this pin rides in can elongate and then you have issues with engagement of the gear. How people have repaired this is something I need to investigate.