Hi. It's good to see another West Aussie Trainzer here.
Unfortunately, the West Australian content for Trainz on the DLS is for older versions (dating back at least to Trainz 1.3) so I'm not sure if they will even work in Trainz 12 (most of it works well in Trainz 2010 in Compatibility Mode, but a couple of items are still broken) without fixing them, a time-consuming task.
Additionally, I'm not aware of any Trainz 12 - specific WA content available at the moment, but that doesn't mean there might not be anything possible in the future.
If you still want West Aussie content regardless, there is too much to list here, but if you search the DLS, make sure all boxes back to Trainz 1.3 are ticked to get the most results, and search for 'WAGR', 'Westrail', 'Western Australia' and 'Transperth'.
Finally, you might want to try looking on the Trainzone website (
http://www.trainzone.co.nz/index.htm) as there are plenty of narrow gauge items (like track etc) which are very useful for a Trainz user working on WA prototypes (if you look hard enough, you might even find a WAGR steam loco or two there as well

).