The networking code nearly reaches the first stage, where people can connect a game and play over the network on the same map. This long term effort is now coming close to functionality. You need a recent nightly and can then start a server by putting -server on the commandline. In the client, you need to enter net:127.0.0.1 as name for the game to load (if you run both on the same computer).

This is still early code, so the nightlies actually may become instable for a little while.

Links:

ternational Simutrans Forum