I wish we'd had this around to research the area our fictional sub is in. Once downloaded, it's a great tool. Click "railroads" in the menu on the right and the purple lines showing basic rail lines magically appear!
If you aren't running XP, Google Earth tends to run really slow on your computer! I usually just use maps.google.com when I'm at work (on this W2K machine) and Google Earth at home (on my XP machine).
Also, you might want to try your local tax assessor's web site for aerial photos and tax maps. They can be very helpful documents to have.
Of course I would be remiss if I didn't mention topozone.com. Excellent information there, but you have to buy the images from them (unless you can figure out a way to copy the image you see on your screen).
