This doesnt seem to have been announced in many places - but potentially it is a breakthrough and I think some people may find it very intersting. The guys over at BVEstation have been working on a multiplayer system for some time, and they have now come up with a sort of multiplayer system. Basically, it extracts the positions of each player (it allows up to 6 atm), and creates what is basically in cab, three aspect signalling. The downside is that you have to run in windowed mode and you can't see other people, as well as the fact that it doesnt sync with signals on the route - but it still has the potential to be very fun!
Their address is at http://www.bvestation.com, and on their forum there is lots of information available about the client. They are working on upgrades to allow fullscreen and ingame chat, as well as for you to see other peoples trains moving!