FWIW, we had a prior discussion on extending one of the Carbondale trains to Memphis last October in the Amtrak Success Stories thread (at pp. 27-28):
http://www.railroad.net/forums/viewtopi ... &start=390.
In regard to the suggestion of establishing Thruways from Memphis and Nashville to Carbondale, Memphis makes some sense, but it is (if you closely parallel the CN route through Newbern and Fulton, with possible stops at each) over a four hour ride, which may be pushing it a bit for a feeder bus. Some miles/ time could be save by going via through Arkansas via I-55/I-57, but it's still a rather long bus ride to connect to a train. It's a fairly similar story from Nashville. It is interstate most of the way, but there would likely be stops in, say, Clarksville, Hopkinsville and Paducah. With stops it also would probably be around a 4-hour trip. Again, rather long for a feeder.
A Thruway route that seems like a no-brainer to me would be Carbondale to Paducah. It's just over an hour (68 miles). While Paducah has only 25,000 population (it appears that a long period of decline may be starting to bottom out), McCracken County as a whole is 65,0000. Its airport hosts only two scheduled flights a day (both to Chicago, both on RJs), and I don't think there's any scheduled bus service. While I don't know much about Kentucky law, if the city/county were interested, I suspect they could fund the low cost of the bus route without the state of Kentucky having to do anything (possibly in cooperation with Illinois--maybe a stop in Illinois somewhere between Paducah and Carbondale).