Just want to mention my original suggestion of Chicago-Memphis was predicated on the idea of improvements north of Champaign, where 3/day might be sufficient business to merit such improvements, allowing a timetable fast enough to turn the train so it didn't require additional equipment. That may be based on my ignorance of how much time you'd have to save before a single trainset could be used. But I thought mention it. I don't think such a service is worth buying additional equipment for. I only thought it would worthwhile if existing equipment could turn.
It's disheartening to hear the estimate that Carbondale is only sending roughly a busload of people north. My instinct is that Carbondale gets far more Chicago metro students than posters above surmise, but in-state v. out-of-state and Chicago v. downstate student population stats are surprisingly hard to google. This article suggests that they see Chicago and St. Louis as their most important recruiting centers:
https://www.dailyherald.com/article/201 ... /304159840
On the subject of Illinois E-W service, the old Illinois High Speed Rail plan included E-W feeder routes. Even that wildly ambitious plan, with Springfield-Champaign, St. Louis-Centralia, Peoria-Bloomington and Independent Quincy, Quad Cities, Galena and Peoria E-W lines to Chicago, never envisioned Hannibal train service, instead showing a Quincy-Springfield bus line. Quincy is pretty tiny, and the days when Hannibal was a major tourist destination are long gone.
I don't know that Chicago-Memphis is feasible. But you've got a relatively big city with a tourism draw and one of the largest, most rail-friendly metros in the country, with some long-time cultural links through the Great Migration; big student centers along the way; and existing corridor service much of the way. If Chicago-Memphis isn't feasible, I don't see how any other Midwestern corridor is feasible.