Known build numbers for TS12:
Build 46957 - Planet Auran release version
Build 47059 - Planet Auran release version (minor revision)
Build 47103 - Steam release
Build 47452 - Just Trainz release version
Build 47772 - Just Trainz release version (minor revision)
Build 48054 - Defunct due to corrupt patch file
Build 48249 - First hotfix
Build 49922 - Added support for Aerotrain DLC - current build
If you have any of the boldfaced build numbers, you can patch up to 48249. Assets with trainz-build 3.6 will then become available.
If you have 48249, you can patch up to 49922. The Aerotrain DLC will then become available.
If you have 47103, you can neither patch nor use Multiplayer. Also, assets with trainz-build 3.6 must be obtained via FTP, then back-dated to 3.5.
If you have any other build, you may have a pirated copy of the game.