Gerd
Thanks so much for posting the photo. It's a big help to me in building the Shay kits. However, unless one of the sliding line shafts is allowed to "slip" inside its mate, then the Atlas system still has a mechanical "closed loop". Both trucks are locked mechanically to the motor and each other through the motor/worm drive shaft. Both trucks are also locked mechanically to each other through the line shaft system as well, although as you point out, only to one axle of each truck. In the HOn3 models, this closed loop causes binding when gear tolerances allow the two drives shafts to get slightly out of synchronization with each other.
My guess is that the Atlas N worm gear drive has much more "slop" than the HO systems (which have universals to keep the worm aligned with the worm gear as the truck turns), and that this provides sufficient tolerance for the inherent "slop" in the line shaft gearing.
In any case, thanks again. I appreciate both your modeling and the pictures.
yours in Sah Shaying