At some point after STL-SGF is implemented, would it be possible to extend that train to Joplin?
Yes, in theory. Of course we're still assuming STL-SGF is going to happen. And it may: MoDOT was much more positive in the meetings last fall than they were in 2007, for a couple of reasons which I'll address below. But first, there are two scenarios in which we may see Joplin service: 1) an SGF-KCY train would easily accomodate Joplin. The most direct route from Springfield goes northwest along the BNSF to Lamar, then north on MNA through Nevada to Harrisonville and then on to join the UP tracks just east of Lee's Summit. However, there are a couple of alternative routes which come up through Joplin, albeit necessitating some connector tracks. One would use BNSF to Aurora, then MNA to Carthage, then following MNA north same as above. This routing does not lend itself to a downtown Joplin station, as it would require some sort of push-pull manoeuvre. The better, yet more complicated alternative would be BNSF (Frisco mainline) through Aurora and Monett, then switch to KCS to southern Joplin, then switch to MNA into downtown Joplin, and north as above. Yes, it's also possible to use the KCS tracks JOP-KCY, but that line is much more sparsely populated, and a stop in Nevada is highly desirable, since it is the regional town for the farm villages of western Missouri. So, needless to say that's a project that's going to take a mountain of work to come to fruition.
Second, if Oklahoma gets serious about their OKC-TUL route, a JOP connection point would be ideal for switching between corridor trains, in much the same way MO and IL do now at St. Louis. Unfortunately, the Frisco mainline ran through Neosho, a much more important town than Joplin at the time. So getting Joplin service on a potential STL-OKC corridor would still require at least the cooperation of BNSF and MNA, and probably KCS as well.
I recall seeing (and reading) the MoDOT study of St. Louis-Springfield service from a few years ago. Has MoDOT released a study of Omaha-Kansas City-Springfield-Memphis (or some segment thereof) service?
I have also read this study. The short conclusion on my part is this should not be done. It is simply too slow. The proposed one train a day also adds to the lack of chance of success. Simply put, this schedule is very similar to the old City of Memphis between Memphis and Nashville which was down to one coach before going away all together in about 1958, and that when the alternative was two lane US 70 with a drive time not much faster, if any, than the train.
The existing railroad alignment is simply too crooked to achieve much speed, no matter what sort of maximum speed limit it has.
I have also read the study, and there were outlined a couple of main issues with that routing that are now resolved. One, there was in 2007 no universal crossover at Kirkwood, which would have required skipping KWD and running on mainly 10MPH track the whole way into STL. That crossover was installed in 2009. That alone shaves about an hour off the time. Secondly, the impending speed increase on the MORR (which the STL-SGF service would share track from Pacific to Gateway Station) means the STL-SGF train could potentially run at 90MPH the last 35 miles of its journey. So rather than just over 6 hrs. from end to end, we have right about 5 hrs. Still not great--but look at this. Now that we're looking at 3.5 hrs. STL-CHI, an SGF-CHI trip on Amtrak runs 8.5 hours--almost exactly what it takes to drive.
In conclusion, given the major upgrades in both Missouri and Illinois, the ridership numbers in the 2007 study can only be taken as accurate for a service that would have begun in 2007, and not for a new service starting now or in the next few years. Granted, Missouri will still need to decide that they wish to pay for this route, and it still will cost us quite a bit to get it started. Other issues like the lack of any usable stations in Springfield, Lebanon, Rolla, and Sullivan are still true today. This route is just a hair closer to happening than it was 5 years ago--and you are correct that MoDOT's highest passenger rail priority is in increasing speed and frequency on the MORR. STL-SGF service, if it happens at all, will not occur until those MORR improvements have already taken place.