Dan, first it is nice to see Craig jumping all over your post and being proactive to reach out to you. Secondly, I purchased 3 sets of the 5 units spines, and all needed a bit of tuning before they were ready for operation. Here is my experience:
I had a few cup holes that were too small for pins - resolution: drill hole larger with sharp hobby knife
I also had some loose details - resolution: glued back on with CA
I found a few wheel sets that were out of round causing the cars to wobble significantly, so these needed to be replaced
I had one car where the end unit had a truck that would not set on the rails level. One axle was lifted off of the rails when the other was on the rails. This appeared to be a bolster screw misalignment perhaps when it was assembled. it would derail before being fixed
one of the end units had the steps and walkways assembled very poorly and misaligned. I too haven't messed with it as I don't want to break it.
For the most part these are minor, but do require some attention to have a car ready for operations.
HuskerN
www.nscaleaddiction.blogspot.com