I've been having similar issues as described above with both my CS and TPE sets derailing over Peco Code 100 Pointwork. Unfortunately its hard to check with the back to back gauge I have as it won't slide over the drum for the pickup in the centre of the axle, and certainly can't use it to squeeze the wheels in as I have with other stock previously. Sounds like I need to get some digital callipers so I can check the measurements that way, but based on several comments above reporting the same issue it seems probable that the back to back is the issue.
That would be great service. Unfortunately I reported issues with my CS set two and a half weeks ago. After the first couple of responses I received felt like a bit of a rebuff I requested to return the set for exchange, after which I got some replacement bogies. Very simple to swap over, so I was perfectly happy to be sent replacements to swap, and they certainly improved the situation. Unfortunately by removing the most problematic bogies it has now revealed issues on other axles. My TPE set has since arrived with a similar issue which I duly reported and have to receive any response at all.
I've also found the 6 coach Aberdeen set to be very draggy, far more so than my TPE set, which was leading to the magnetic coupling parting. I've partially resolved this by lubricating all the axles. The rolling resistance is still fairly high, but at least the couplings don't part, and the Class 92 is more than a match for them!
It does seem to me the the rolling resistance increases after the stock has been running for a while, maybe 10mins or so. I was wondering if this could be due to the axles warming up and expanding slightly? But not really sure; has anybody else observed this?
I'll be persisting until I get reliable running with both sets, they are head and shoulders the best looking set of coaches on my layout :-)