... use the existing stations at Marshall (albeit with a new second platform), Jackson, Meridian, and Birmingham ...
This reduces the problem to:
(1) agreement with UP for Dallas-Shreveport ...
(2) agreement with KCS for Shreveport-Meridian
Wikipedia suggests that the Meridian Speedway is in good shape and underused:
In 1993, KCS acquired the line . . . 2005, KCS and NS announced . . . a joint venture. KCS contributed the rail line, NS $300 million in cash, almost all of which was slated for
capital improvements to increase capacity and improve transit times. . . . By September 2007, about $135 million had been spent on the improvements. Several new and longer passing sidings were installed along with a new CTC signaling system. The mainline was effectively rebuilt from the ground up with new ballast, crossties and heavier welded rail. Soon after this first round of improvements, about 45 trains per day traversed the line.
Today the line sees fewer than 15 trains per day, mostly run-through Norfolk Southern/Union Pacific intermodal trains.
(3) agreement with NS for Meridian-east (they will *definitely* demand a new Atlanta station . . . )
No "day train" will come to ATL until it gets a working station. LOL.
(4) station funding for Shreveport and Vicksburg . . . Ruston and Monroe would be bonus
The casino interests will make sure a nice station is ready in Shreveport.
You're overestimating Vicksburg for historical reasons, and that could attract a disproportionate number of tourists. But it's about the same pop as Ruston, without a big college like Ruston's Louisiana Tech. Greater Monroe is 6 or 7x as big as Vicksburg, plus thousands of students at a Univ of Louisiana branch campus and at nearby Grambling State.
But it seems local funding (plus TIGER grants) is often easier to get than any help on the tracks, so I doubt that stations will be a stumbling block.
(5) funding to buy locomotives and rolling stock
Where's that 70-car CAF option when we need it! Might have to use rehabbed Horizons.
(6) any track & signal improvements needed for decent speed, or demanded by the railroads
Again, Wikipedia indicates it won't be much if any needed west of Meridian. Make sure the stations are on sidings not to block other trains. Build a couple of 10,000 ft sidings and good to go.
East of Meridian, and thru the tail end of the Appalachians, someone could make a persuasive case for tunnels, oh, boy.
(7) operational funding
Texas and Louisiana might well fund that Dallas-Shreveport corridor that largely overlaps the Eagle. That segment will show an operating profit on weekends. LOL. We'll see if the Pelican State puts in a million or two to extended the CONO eastward from New Orleans. Heck, we'll see if Mississippi chips in. If they do, that would set a nice precedent. And log-rolling would begin: "The state is paying for your train, now vote to pay for our train." Alabama and Georgia should help cover costs for the Atlanta-Birmingham corridor. And of course, it should be simply part of Amtrak's budget for the national system.
This route has been talked about for years. It ain't going nowhere until after the CONO sans chef is up and going. Then the Southern Railroad Commission can turn its efforts to this route and we'll see.