In Trainz Simulator 2 Mac, all of your downloaded and custom content is stored in a 'data' folder in separate to Trainz itself. To find the 'data' folder, please follow these steps:
1) Open Finder
2) Navigate to the following folders: Macintosh HD -> Users -> (Your Mac Username) -> Library -> Containers -> com.n3vgames.trainzmac2 -> Data -> tz
3) You will now see a list of folders shown, such as 'local', 'original', 'packages', 'editing', etc.
The 'original' folder contains all unmodified content from the Download Station.
The 'local' folder contains all content that is modified, that you have created yourself, or that has been installed from locations other than the DLC system or the Download Station
The 'packages' folder contains all payware content that has been installed via the in-game DLC system.