Had been passively following this thread since the summer, but had to join today to bring this to everyone's attention:
Looks like a possible fix just posted to Nexus over the holidays by author Syyyke: Syyyke's RDR2 Spawns FIX
https://www.nexusmods.com/reddeadredemption2/mods/5495?tab=files
It seems to work for me thus far after about 10 hours of play time. Was shocked at the number of trains I could HEAR before even seeing them. Complete change in terms of ambiance.
Only hassle is it "requires" ScripthookV2 (instead of AB's...he has an explanation for why as it relates to the treatment of the Ped pools), and dinput8.dll (instead of version.dll). Some mods may not work if those are swapped. Importantly, he attributes version.dll to being part of the reason game mechanics get borked.
Perhaps the best news is threefold: (1) he has been actively working on this since November 2024; (2) most feedback is very recent and positive; and (3) he is quick to respond to feedback, questions, and troubleshooting. Looks like we finally have someone who is aggressively working on this in real time!
EDIT: as I posted this, he just updated it version 2.1 and included a handful of patches for popular mods (looks like he is taking requests for new patches, too!).
EDIT 2: It is now a "Hot Mod" on Nexus.