I have to be courteous enough here to respond to the various comments trying to help.
Lima asked "Does the front tender truck rotate freely as well?". ANS: yes, it does. The rolling stock (cars) are very short as well, 6 or 7 inches long tops.
TinplateBob: "I'm 99.9% sure your problem is the latch lift pin sticking out the back of the coupler.". ANS: I don't know how to prove that though. I hate to "break" the tender's mechanism without being sure, even tho' I never remotely uncouple. I'd love to know how to find out for sure.
Mike: "You could be a bust in the Hall of Fame!". Yeah, I'm headed there with this tin-plate stuff! I really am regretting selling all of my 'O'.
Michael R: "but is the tender empty? Would it help to put weight in the tender?". Mike, this is a PS3 set and the speaker and boards are in the tender. It's rather light but not light like it's empty.
Scott: ANS: My tin-plate 2 sets are both modern LCT sets both PS3. no old stuff.
hojack offered: "ow it's just a matter of applying the requisite tweak and you'll be rolling smoothly". ANS: agreed but knowing what that tweak is doesn't seem easy to determine and the tweak itself may not be easy either. But since I don't know, with 100% certainty, what that tweak is.........
PCR: the track is MTH's RealTrax. I have a bunch of it still from starter sets that I bought that were Railking 'O'. I don't know what it is, 36"??? The figure 8 that I set up uses some of the new and some of the old, both the same diameter though.
I'm going to set up an '8' using 42" curves since that's what will be on my Christmas layout. It BETTER run OK on that!!!!! BTW: I also tried my 'O' Baby Blue Comet and it's OK with the MTH RealTrax. Funny because it's way bigger, cars and all, than the Christmas set (both are 2600 series stuff I think that's the right term).
thanks to everyone for offering ideas, one of which hopefully eventually is the answer. TinplateBob, I'm not doubting what you offer, it's just that if I have to sorta ruin the coupler to find out I don't know how willing I am quite yet to take that chance!
as always, thanks - walt