One great improvement during the Lionel MPC era was the new fast-angle wheelsets with needle-point axles that roll much better than the old postwar wheels. But it's curious that they came out with the Symington-Wayne style of truck, which I understand was never used on real railroads in any quantity.
I've seen comments that MPC's rework of Lionel's AAR roller-bearing truck was somewhat unsatisfactory, perhaps due to old molds.
I accept older Lionel trains as being somewhat whimsical and I kinda like the Symington-Wayne trucks as being different. I'm speculating that MPC Lionel chose the different style with the idea that they might pass for freight or passenger trucks, or as something different or modernistic. Or maybe there was a possible issue with patent infringement if they copied a conventional design??
I picked up seven pairs of fast-angle wheelsets for cheap at a recent train show, most of them S-M trucks. I'm going to refit them on some of my postwar freight cars because they roll so much easier and I like to run long trains.