In the Player List, you would be given the opportunity to mark each other player as
You can then operate a vehicle into any station where the owner has marked you as an Ally. (The Public Player would mark all other players as allies; you would be your own Ally, so there is just one check required to see if you can serve a station).
You would be forbidden to build inside the city limits where another player has built their Headquarters (finally giving that a function in the game)... although you could operate vehicles across public roads through it, or stop at an Ally's stations.
The AI players could then, for example, set as Ally any player (human or other AI) that uses a mutually exclusive set of modes (all rail versus all ship), or set as Competitor any player that builds a station inside city limits where it has stations.
I think I'll quit playing Simutrans. It's becoming too much MMO-ish. ;)
(o wait, I think I've said this before! :o )
/irony