To properly backup or restore the Trainz for Mac database and config, you should backup the TrainzSimulator folder in your user's Application Support folder. It's located in your user's Library folder. If you are running Lion, this folder is hidden.
To access it in Finder:
Cmd-Shift-G
Enter ~/Library/Application Support in the field
You can then right-click on the folder and select "Compress TrainzSimulator" from the popup menu. Move the resulting TrainzSimulator.zip to a safe location.
From the command line:
cd ~/Library/Application\ Support
zip -r ~/TrainzSimulator_Backup.zip ./TrainzSimulator
Make sure that Trainz is not running before you back it up or restore it.