Help/Driver/Main Menu/Options
From TrainzOnline
Clicking on "Options..." will open up a menu which contains tabs for choosing different settings. To access each of them, click on the highlighted rectangles to the left. Each tab contains squares next to a description of a setting. A square with a green tick in it means it is currently enabled. To disable a setting, click on the square next to it, the tick should disappear.
To the bottom right corner of the menu there are two symbols. Pressing the "tick" will save the changes made. Pressing the "x" will cancel the changes.