What prevents us from getting open borders and spamming railroads on AIs territory to tank their economy?
Nothing, except the AI can just rip out the roads as soon as you build them.
You territory will reach 4/5/6 hexes from your high-cultural cities (like your capital). It naturally claims tiles towards luxs and resources, but takes a while. Some people claim you can influence the border growth by having units stationed on hexes you want, but I have never had success with that!
Other than that, you can use a great general to build a citadel within a hex of the resource.
If you build a citadel, all the tiles next to it become within your borders. So you don't need a road. Oil rescources on sea can be worked with a work boat but you need Refrigeration to construct a well.
Still cannot figure out how to get notifications back. I have to bring up the notification log to find out what Pantheon was founded or if a Wonder was built. No idea how I lost these notifications in the first place either.