Google Earth, Yahoo! Maps satellite view, and other public-accessible satellite phot websites are invaluable for this. All you have to do is go to Yahoo! Maps, take a look at a city with lots of rail service, switch to Satellite view, and follow the rails. Often city maps will have simplified schematics of rail systems. Depending on the specific railroad, there are many books on railroad history and operations which often include track plans, and historical societies and railroad museums often have repositories of track diagrams and maps. There are a lot of different sources, rather than a single one, and they vary based on what railroad you want to model. Studying real railroads is the best way to get inspiration for model railroading.
Another suggestion is that if there are trains operating in your area, go out and do a little railfanning! Take a look at how it looks from the ground.
Gavriel609 wrote:I am wondering where modellers actually find the track plans for the prototype. Is there some sort of information website or something?