This map, from the American Intercity Bus Riders Association, attempts to show all the major intercity rail and bus routes in America. It includes Amtrak, Greyhound, and several other bus carriers.
Colorado may not have a Front Range passenger rail line yet, but it turns out you can actually travel to a lot of places without a car.
It’s probably impossible for this kind of map to be 100% accurate all the time. In all likelihood there are some missing links, and missing carriers. But it’s still quite an impressive undertaking, and a useful tool to bookmark.