What I did:
1. Installed RSO.
2. Generate a new Railway map without changing any settings.
3. Moved a save to the server.
4. Added RSO mod to the server.

After joining, I see a map without RSO generation, but in single, it works.

Did I do anything wrong?

{
"mods":
[

{
  "name": "base",
  "enabled": true
},

{
  "name": "helmod",
  "enabled": true
},

{
  "name": "rso-mod",
  "enabled": true
},

{
  "name": "Warehousing",
  "enabled": true
}

]
}

Okay, uploading temp/currently-playing/* from the local machine to the server helped. Is it because of level-init.dat file?

I'm sorry I have no experience with headless at all.
You might get more info on forums.