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I had to change the values in the trainzoptions.txt file to get my screen format correct. This is in addition to the display setting in Configure Trainz (which I have set to 1600x1200).
For what it’s worth the trainzoptions.txt settings for my wide screen format are:
-height=1050
-width=1680
I arrived at these values after a bit of trial and error.
You probably already know this, but if you have difficulty in finding the file in TS12 there has been a change of location. To quote James Moody,“..trainzoptions.txt is now an optional file (not required), and should reside in the UserData folder, not the main installation folder”.
Another (unlikely) thought which might be worth trying first is the F11 key which is the Windows toggle for full screen.