Check out Open TTD. It is an overhaul of Transport Tycoon Deluxe with updated graphics. It is open source (read: free) so installing across a network won't strain a budget. I haven't personally tested the multiplayer function but this is a very active project, so problems are constantly being addressed.
The only problem might be the complexity of the game, as it has a bit of a learning curve. The wiki provides good tutorials that range from basic to overly complex however.