Created an account to comment on this topic as I'm also huge in modding RDR2 (played on/off for years so most older mods I have before I still use them now unless a newer mod was developed). Probably like everyone else I also got bothered with the ambient spawn stuff when I read everything in this thread so I tested it and performed some cleaning on my end. With lots of mods and stuff in the stream folder and specially WhyEm's DLC I noticed the very issue that this topic was talking about (no Trains, no passengers in Trams/Carts, no pink spoonbills).
Not going to claim any solutions here since I didn't thoroughly tested the train spawning part like Dagger explained here but when I removed WhyEm's DLC and most of the stuff in the stream folder (specially player model changes) I immediately saw passengers in Trams/Carts, some Trains (Valentine, Saint Denis, Scarlett Meadows Crossing, Bridge in Riggs Station) and the rare spawns west of Saint Denis (pink spoonbills). I downloaded Railroad engineer to know the Train Spawns and even with lodscale Adjustment (1.0-1.25) and Ambient spawning mods (like Ambient Random Events, Ambient Gangs, Improved Wildlife and Hunting Grounds) I can see some trains now unlike before (again not fully tested as posted in this thread but seeing them now was a huge improvement). Honestly I'm fine with the results already and I just wanted to play the game again lol since I already spent several hours cleaning and testing and finding the compromise or balance I'm looking for with my mod loadout.
Not sure it'll be helpful for others but pretty happy with the mods I have right now (still quite sizeable after some clean-up) so sharing them to you below with some comments on changes I tinkered with in the mods (some in systemsettings and globalconfig too like lodscale and ambient/max peds value).
Scripthook: February Maintenance Version (I found this Scripthook stable for most older mods and some newer ones, besides this is the only Scripthook that Rage Plugin Hook works with as I also play RDRFR)
ASI Loader: Use the one in the Scripthook V2 (dinput8.dll + OCU, I find version.dll very buggy so YMMV)
LML: Latest LML (Beta 11)
ASI mods:
Alder Ranch.asi (use this only in epilogue)
AmbientRandomEvents.asi
AMJMTransport.asi (older version as latest doesn't fully support controller inputs)
AuroraBasinCabin.asi
Bandit_Hideouts.asi (random Posse attacking enabled but disabled Roaming bandits)
Banking.asi
Beecher's Hope Dialogue Enhanced.asi (use this only in epilogue)
Boats.asi
Boat_Animations.asi
BountiesExpansion.asi (Older version before the Bounty Wagon was introduced as I found this version most stable, also disabled random events and BH's)
BucketWash.asi
CampAnywhere.asi
CampfireWeaponLocker.asi
Companion.asi
Contracts.asi
CoresRegenOverTime1.0.asi
CutDialogueRestorationandEnhancement2.1.asi
DogCompanion.asi
EasyHookPatch.asi
FastWalk.asi
GatlingWagons.asi
Graves.asi
Gunsmith.asi
GunTricks.asi
HorseLoadout.asi
HorseSprint-v2.asi
Houses.asi
HuntingCamp.asi
HuntingGrounds.asi (disabled every feature except for the Dynamic Size as that's just the feature I wanted in this mod)
ImmersiveGuarma.asi
Injury.asi
LockdownManager.asi
LOS Enemy Blips.asi
Minimap.asi
MorePersistentWeather.asi
Needs.asi (older version)
NoAutoEquip.asi
NoAutoReloadEver.asi
NoDeadEyeEffects.asi
NoDualWielding2.1.asi
NoHolstering.asi
NoSinking.asi
NoTwinRocksThievesLandingScripts.asi
Outfits.asi
Pay.asi
PedDamageOverhaul.asi (Light version so I can use it with Ped Accuracy overhaul)
Rampage.asi
dr2-translator.asi
RedDeadLawmen.asi
RestoredCutContent-Quiver1.6.6.asi
ShavingKit.asi
SleepDeprivation.asi
SmokingComplete.asi
Softer.asi
StashThatLantern2.8.asi
StashThatWagon.asi
ThirstyHorse.asi
UnlockCores.asi
vfs.asi
WeaponScope.asi
whm.ammunation.asi
whm.pedaccuracyoverhaul.asi
LML Mods:
Ambient Gang - RDR1 Gang
AuroraBasinCabin
Contracts
Corpse Spawn Distance
Follow People
Graphical issue fixes
systemsettings.ymt: lodscale 1.0-1.25, pedlodbias 0.9,
timecycle_mods_1.xml: slod_fade_power 0.000 0.000, lod_mult_slod1 0.900 0.900
HappyEndingMoodAddon
Herbalist
Holstering animation 1.2.2
Huntsman
Immersive Scenarios
Improved Wildlife
John Can Swim
JustShutUpAlready
JustUseTheCrossAlready
LAW
Maverick Weapons
MaxPayne (this is the "An Experimental Game Config" Mod, I used "Stutter Fix" mod before I found this, the changes are already there anyways for the 8GB VRAM GPU I have, there are still pop-ins but you gotta find compromise in the end lol)
gameconfig.xml: (AmbientPedsMultiplier = 150, MaxTotalPeds_Base =150, MaxTotalPeds = 250)
no_ammo
Online Content Unlocker
PDO v2.0 Extended Features
Perspective (w/ some changes for my personal liking but I don't think we need to mention here as it's not critical for the ambient spawn issues)
RedDeadLawmen
Silent Deadeye
Thirsty Horse
WickedHorseMan/Gameplay/Timescale (was using the "Consistent Timeflow" mod before this mod was uploaded)
Stream Folder:
hud_reticle.gfx
rpg_textures.ytd
a_c_horse_tennesseewalker_mahoganybay.ymt
a_c_horse_turkoman_darkbay.ymt
a_c_horse_turkoman_silver.ymt
a_c_horse_turkoman_gold.ymt
water-seiska.ymap
frontend_store.ytd
a_c_horse_mustang_grullodun.ymt
a_c_horse_buell_warvets.ymt
a_c_horse_arabian_white.ymt
a_c_horse_john_endlesssummer.ymt
a_c_horse_mustang_wildbay.ymt
a_c_horse_mustang_tigerstripedbay.ymt
hud_toasts.ytd
a_c_horse_dutchwarmblood_sootybuckskin.ymt