Dude, I think I went through the same thing and pain you did, ha-ha. Anyway, I decided to post my dev drafts to the public publicly, as I ran into some limitations/bugs of RedM Onesync and stopped working on the project, at lease until my bug report is in idle state:
Here are my findings and almost working script of server-side persistent trains:
https://github.com/draobrehtom/redm-trains