From the Trainz Launcher, select Options, then select the Display Settings tab, there should be a few choices for Display Resolution. Be sure Full Screen is unchecked if you don't want Trainz to use the full desktop area. You may also need to change the Aspect Ratio. (1024x768, 800x600, 640x480 are 4:3 aspect, and 1280x1024 is 5:4 aspect ratio, and 1600x900 is 16:9 aspect ratio)