"I don't understand how you go from the having best trucks and couplers, to the worst."
"I don’t know why the original tack-less design was abandoned- those actually tracked better and had a spring suspension."
It's very simple:
Old design = all metal, spring suspension (more difficult to assemble) = expensive
New design = lots of plastic, no spring suspension (easy to assemble) = cheaper
Lionel's goal is to make money and providing cheaper trucks accomplishes this. Very few of their customers couple and uncouple cars. Most run the same train of cars on their loops or sit the cars on shelves to look at (or keep them in boxes in closets).
Maybe they will modify the new design to be compatible with the old one. We'll see.
