I think someone should make a clear guide on how to solve the problem yourself with the methods that have been found so far (texture compression, install.xml replacement method, etc...), or at least a summary of the findings, since a few days ago I read the whole thread but it's very long and a bit hard to digest mentally to understand what to do (besides being very time consuming). And since it's very long the important things are scattered and hard to find, even though I read it all.
I'd do the guide/summary myself but I don't know how to make the install.xml with the replacement method instead of the streaming one, but I understand why streaming the mod files causes the problems.
I also have problems turning some .dds files back to a .ytd using RDR2TextureTool. I know there's a solution but I can't find it explained clearly.
This is a fast and easy method I use for checking if the mods are causing spawning problems:
I use this clean Ch. 2 save file that was referenced in this thread but not linked yet (I think), with Rampage and Railroad Engineer installed: https://www.nexusmods.com/reddeadredemption2/mods/4015
(For newcomers, it has been said that save files could get corrupted in some way by some mods, so it's better to use a clean save.) I recommend disabling autosave if you plan on doing many tests.
After loading I open Rampage and go on "World" and activate the ESP in "Ped Manager" and "Vehicle Manager".
Then I activate No Clip (in Miscellaneous), set max speed and go check if EVERY train shown on the map spawns and if ped spawns are normal too on trains and trams.
Since you move very fast at max speed with No Clip the train might not spawn at first due to this. If that's the case then wait a few seconds for the game to load the area where you are, then fly away from where the train should spawn while looking at the railroad, after moving away the right distance you should see the train spawn.
This method works 100% of the time for me, and could be included in the guide on how to solve the problem if the experts here agree that it's good (because I'm no expert), since it's very quick and easy.
I also read that graphic settings could contribute to the problem, I play in 1440p with high-ultra settings.