Bryan, you and Beachhead and I all seem to have a similar want for really good detail. I've actually studied this and thought about it as I've bought new and used locos over the years. I do not want to claim to be an expert, but here is a summary of what I think about this topic:
But first, a caveat: I like to see crisply molded rivets, panel lines, etc., and love a loco, particularly a steamer, with lots of those 'separately added parts' for pipes,ladders and climbing handles, valves, levers, etc., along with operating cab windows and roof hatches, etc., etc. But fI am not a real stickler for prototypical accuracy: I don't really care if the loco has 53 rivets on a particular panel when the actual loco had 57, etc. - as long as they are good, crisply-molded rivets.
So, back to detail. I think over time, MTH, Lionel, and to some extent WBB have occasionally added detail to a new release of a model they have issued in the past. The best example of that is no doubt the scale NYC Hudson J or the Niagaras made by Lionel. Lionel has made a scale Hudson since 1937 or so and issued new models of the Hudson every so often since, and reissued its Niagara several times, too.
Now, not all of those "re-issues" had new, better detail or features, but over time Lionel occasionally added 'stuff' to their scale Hudson so that the most recent release is about as detailed and feature-laden as anything on the market and definately ahead of anything done twenty years earlier. MTH has done similar with locos it releases at intervals, too, such as the Santa Fe 2900 Hudson - I had one from the PS1 era and the most recent PS3, and the latter is noticeably better than the former (but its still pretty damn good!).
Thus, over the very long run one can generalize that "Usually detail on locos made recently is better than detail on those made long ago."
Point 1) But that isn't a hard and fast rule, there are exceptions, and my working assumption unless I know and can see otherwise is that a newly released version will not have more detail than past versions unless the manufacturer says it will. Usually, in the catalog, the detailed 'fine print' will say something like, "new added detail to firebox and boiler piping, and cab windows' or something like that.
Point 2: I suggest not correlating the detail (and features for that matter) with the electronics, by which I mean PS1, PS2 or PS3, or, for Lionel, so-old-it-has-no-TMCC vs. TMCC vs. Legacy, vs. Legacy-with-bluetooth. I've seen no strong evidence that Lionel or MTH or others say to themselves, "since we're improving the electronics, lets improve detail too." Frankly I think its the opposite, "We're issuing this again in Legacy, so lets give them more detail this time so they have a reason to buy the newer loco."
Instead, look at the periodic re-issues of the loco by year. Both MTH and Lionel have released versions of the same loco two or more times under their current PS3 or Legacy systems, but with different levels of detail added. I think this is the norm, not the exception.
Point 3: I avoid PS1 locos whenever I can because of problems with the electronics and because PS1 sound is generally not as good as more modern locos, not because detail may not be good. But I have several old PS1 locos that I have purely as shelf queens that have particularly good detail. One example is the original PS1 UP coal turbine. Mine does run, and I got a bargain on it when I bought it because the PS1 electronics had fritzed. But it is a nicely detailed model and serve that purpose well for years.
Similarly i avoid early TMCC locos if I can because they don't run as well in conventional (what i run) as legacy.
Point 4: From personal observation, I have noted that manufacturers almost never remove detail in later issues - but this does happen. I can't recall who and what now but one steamer issued in circa 2008 had an operating cab roof hatch that was, on the updated latest 2018 release I bought, cast in place rather than a separately applied moving piece as in the one I had from around 2008.
Point 5: Finally, again from personal observation, I think that detail varies alot between different model locos more than between say, PS2 and PS3, or between those issued in 2008 and 2018. Some locos just are detailed, others less so, and the manufacturer seems to be happy to leave it so or only improve it a bit when they re-issue it. An example is MTH's Santa Fe 3460 series Hudsons. The latest issue, in PS3, from maybe 4 years ago, is extraordinarily detailed, matching anytying I have seen 3rd Rail make. I understand that its really only the same level of detail on an older PS2 version - that loco has apparently always been made with really good detail several time in that last 20 years. On the other end of the scale, Lionel's perinnial 0-4-0 A5 has "barely adequate" detail in my opinion and apparently has had that same level of detail since it was first made.
Paying doo attention to photos on the internet can help you identify locos that will be detaied. But going to train shows, or the club meets, and paying attention or asking when you see a particularly good looking loco is the way to find out who made what, when.
