All of my recent Lionel steamers are noticeably smoother at low speeds than any of my best locos from even ten years ago excepting the JLCs. Mallet, V Hudson, V Challenger, SF 3759, Atlantic, Southern Crescent, the Yellowstone, two-truck Shay.
I do not know if this is because all locos lose some smoothness as they age - that could be the case I suppose -- but I doubt it: some of my older locos have only 20 hours on them and have been maintained and lubed properly. And they run very well, just not as exceptionally as the newer.
I think it is both the motors and perhaps attention to the gearing, etc.
There were two exceptions: the recent conventional Mogul - mine ran but ran poorly and I just replaced the entire chassis with one from a WBB 10 wheeler (rather drastic iI admit, but a superb little loco now), and a bargain RTR English 4-6-0 Hall class (as for the Hogwart's train) which has a tiny motor as for a trolley or something in it and shows its low price-point design in shuddering a bit at very low speeds.
