Regarding St. Louis to Memphis, it sounds like Amtrak never had a day train over that route, just section of an overnight train that would have been convenient only for the endpoints and points further south and west. The fact that that service wasn't attractive enough for enough people to object to its elimination nearly 30 years ago doesn't mean that a daytime train couldn't work today. It's around 300 miles and a little under 4:45 via I-55. Could a day train, or a pair of them, get passengers along that route or along a somewhat longer route through Illinois? I don't know, or have an opinion; I just think that a daylight corridor is a lot different from a night run tied to the schedule of an LD.
The important item that needs to be done for a Saint Louis to Memphis day train is to find a state, or a group of states, to subsidize it. Missouri already subsidizes trains between Saint Louis and Kansas City over tracks entirely within the state where the state received all the economic benefits. They are not going to subsidize a train with a mile or two of tracks within the state. There are no large cities in southern Missouri on track corridors west of the Mississippi River. There are no large cities in western Kentucky or eastern Arkansas, they are not going to subside a train that provides zero economic benefits. Tennessee will be hard press to subsidize a train to Memphis while not doing so for Nashville, Knoxville, Bristol, and Chattanooga. So that leaves Illinois, which is willing to subsidize trains to Chicago. But this proposed trains does not go to Chicago, it will die a premature death. So there we are, a proposed train that nobody wishes to subsidize. It will not have a chance to get off the drawing board.
Let’s take this a little further. The largest airline in Saint Louis is Southwest, with 3.8 million boarding, American second with 1.1 million boarding and Delta third with less than 0.9 million boarding. The largest airline in Memphis is Delta with 1 million boarding, Southwest second with 0.7 million boarding, and American third with 0.6 million boarding. So Southwest should be the largest servicing both cities on average. It has nine daily flights from Saint Louis to Memphis all on 737 jets, let’s assume there are just as many in the opposite direction. None of them fly directly, they all stop somewhere else; in Chicago, Dallas, Atlanta, etc. That reflects a general disinterest of business travelers flying directly between these two cities. What makes you think a slower train would fair better?