The problem you mention has nothing to do with dual monitors. The problem is the program is not running at the same resolution that the display is set at.
1st go to your display control panel for Windows, and record your display settings. Should be something like 1600 x 1050, 1280 x 1024 as examples.
Close this, and open up the folder where Trainz TS2010 is installed.
Usually c:\Program Files\Auran\TS2010.
Note: If you're using a 64-bit Windows, the path is similar, except it will start with C:\Program Files (x86)\...etc.
Open up the trainzoptions.txt file.
Enter in the following information.
-width= <-----Put in your width setting such as 1600
-height= <----- Put in the height setting such as 1050
Save the file and close the folder.
Start Trainz.
Open up the Options item on the Launcher screen.
Change to the Display Settings and set this to the same settings as your video card, and the same as those you typed into the trainzoptions.txt file.
Now to get the program to span two monitors, I can't figure this out, and I was hoping your post had the answer.
Good luck,
John