Instead of running CM from the Trainz start menu you can run CM directly from a Windows Shortcut to ...\TS12\bin\ContentManager.exe.
Once you have the Shortcut set up, right click on it, select "Properties" and on the "Shortcut" tab set the "Run" drop-down to "Maximised".
Clicking on the shortcut will then open CM full screen - you can launch Trainz from the "File" menu in CM and when you exit Trainz you will be returned to CM.