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Auran had a Mac version of Trainz being tested during 2006, that was apparently scheduled for release in late-06. Unfortunately, the project was reportedly discontinued according to several online sources, but there was never any official statement from Auran regarding the status of the Mac version.

The latest "official" word from Aural on the Mac version (from a Auran FAQ) that I've been able to find, says:

Q. Will a Macintosh version of Trainz be released?
A. We have not ruled out the possibility of a Mac version of Trainz in the future.

Auran just released their latest ver. of Trainz (Trainz 2009) yesterday. It's Windows (only).
 
Update

In case anyone was still interested in this, Auran released "Trainz Simulator" on the Mac App store December 16, 2011 at a price of $37.99.

Unclear as yet if the assets in Trainz PC are compatible with the Mac. There is a considerable amount of add-ons for the Trainz franchise on the PC side.
 
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