It's mostly texture mods that cause this issue, as well as mods that affect NPC spawns like ambient gangs (I think) but really it's texture upscaling mods Rockstar didn't plan for that mess with RAM allocation ( I am not technical ) and I fixed my issues by not using any texture upscaling mods at all, I have about 35 mods I use for my story and I have no spawn issues at all for trains, buggy passengers, and trolly passengers. Hope this helps anyone