Please note, that the 'trainz-build' is not the build number on the launcher screen, but rather is the trainz-build/version number (e.g. 3.5 and 3.6 for TS12/TSMac, etc). If these match between the same versions of Trainz, then you can join a multiplayer session. We have recently released an update for the Steam edition (several months ago) that will update it to the latest trainz-build number, so as to allow multiplayer to work between it and the 'international' edition available from our store, and a few other locations.
Regards