I’ve sorted my stock into three groups - older, heavyweight O27 stock, MPC and newer lightweight, traditional-sized stock and larger, lightweight modern.
Of the three groups I’d say that some of the best quality pieces are the Lionel O27 size, 24xx carriages. Very nicely done, pickups on all trucks, they track very nicely but they DO take some pulling because if their weight and rolling resistance, and the hot shoes on the trucks tend to cause sparking over switches and occasionally, unwanted uncoupling.
the freight stock is heavy but very robust. Lots of play value, there.
The MPC and more modern stuff is bright and attractive but definitely less robust. It rolls well, and it’s lighter but mixing it with older stuff tends to derail it on curves.
The 1990s-era “small Madison” carriages definitely suffer from quality issues, especially the running gear. The 2000-ish extruded aluminium carriages are rather nice, and much better construction quality.
Moving on to the locos, I’m not going to comment on the PW ones I have, because they all require work. The Williams die-cast scale Hudson seems a very nice piece, in that vein. Of the two semi-scale Hudsons, the MPC era one runs nicely in the fashion of open-frame AC motors - less power and less controllable than the can-motored ones. The K Line one is nicer, but definitely more of a model, less a toy with its larger pilot wheels and more fragile detail. The MTH Premier K4s is again, a nice model but badly let down by its PS1 control system. A definite winner is the Lionel semi-scale PE Berkshire, which tracks nicely, runs smoothly, fits nicely with O27 and traditional size stock, has no fiddly tether and a nicely executed body shell. Very much an “updated PW design” in the manner of the Williams Hudson
turning to the track, as a general comment, the fit and finish of modern tubular track is markedly inferior to the older stuff.
The RailKing controller is just a thoroughly useful piece of kit, and the can motored locos like it (the open frame, AC motors seem less affected by it).
So, there are definite stand-out items, a few definite losers... but I wouldn’t say there was a definite trend in quality vs time, either way. Depends how you define it. There is definitely evolution of design, but that isn’t necessarily the same thing.