I have an HO scale Gordon steam locomotve of the Thomas and friends, series. Lately on every circuit around the track the tender derails in the same place every time.
I 1st checked the wheel sets for the NMRA wheelset width gap,- It was perfect. I checked the RP25 flange height, again, perfect!
 I noted that there is a small downward nub on the bottom of the couplers which tended to kick up when the coupler traveled through through, and over, the switch's flangeway, frogs etc. So, I trimmed them. But, no change, it still derailed! I then tried adding an once of weight to the tender- still derailing! And, finally of note, I have checked the flangeways with an NMRA guage and it is in an acceptable range... Interestingly, Gordon is pulling 4 passenger cars, 4 axels on each car, and nothing else derails including Gordon's pilot and trailing trucks!!! 
Am I missing something? Open to suggestions but this train has run on here many times and this is a new issue.
Thank you!

Baffled

It could be a problem with the tender chassis. I had the same problem occur with my Gordon a few years ago. One of the trucks on the tender chassis wasn't tight enough and as a result, that particular wheel set was allowed to pivot to one side further than the other wheelsets which caused it to derail around tight curves. See if that's the problem on yours.