In Windows 10 right mouse click on the desktop and select Display Settings. You can also get there from the Start Icon (bottom left corner of screen). Select the Settings icon then System and Display Settings should be the first window on the screen. Scroll down (depending on your screen size) to see the Display Resolution value - it will be shown as nnnn x nnnn - it should have "(Recommended)" after the numbers. This is your optimal resolution for your video display. If it does not show "(Recommended)" then scroll down the drop down list to see one that does. Select this value (unless you have a pressing reason not to). Exit the Setting window.
From Trainz Launcher select Trainz Settings. In the General tab tick ON the Full Screen box (if it is set OFF) which allows you to change the screen resolution for Trainz. From the drop down box select the same resolution shown for Windows 10. You can then tick OFF the Full Screen box if you do not want full screen in Trainz (I use OFF when I need to constantly switch between Trainz, Google Earth, Firefox, etc) otherwise leave it ON if you don't want the Trainz Menu bar and the Windows 10 toolbar on the screen. The other setting, Vertical Sync, I leave at Half but others use Full or None. I am never really sure what the differences are and it does depend on what your screen can handle.
Hope this helps.