There are several posts here with tips and links to tutorials. You use MicroDEM to create an image that can be imported into Trainz to give your map the terrain features. Be aware the DEM files (elevation maps) are 30 meter resolution so you won't get totally accurate terrain. You then use MicroDEM to overlay a Tiger file (map) onto the elevation data. This gives you the location of rivers, roadways and of course, railroads. When finished, import into Trainz per the tutorials and you now have a set of baseboards with terrain features and guidelines for laying track. These layouts can be huge, so be aware of what you want to model when you are cropping the final image as that determines how many boards will be in the final .gnd file in Trainz.