ON my system, if I select "options" on the opening splash screen, I get a "Configure Trainz" window with several tabs, one of which is "display settings". Selecting that tab allows me to specify the display mode (Direct X or OpenGL), Display resolution, Bit depth, Aspect ratio, and Antialias mode. Although my monitor is significantly larger, for the reasons cascaderailroad recited, I have the display set to smaller than full screen in that tab.
ns