As said, this topic has been fairly well beat to death.
First, the market was never near as big as the east coast to Florida market. When the major transport was still rail in the early 1950's the East Coast to Florida supported several major trains on the two major lines, Atlantic Coast Line and Seaboard Air Line.
ACL:
The Champion, frequently in two trains, one to the west coast of Florida and the other to the east coast.
The winter only Florida Special
then there were the secondary runs who's names I don't remember
SAL:
Silver Meteor
Silver Star
both of these divided at Auburndale into a Tampa-St Pete and a Miami section.
The SAL also ran a seasonal train called th Orange Blossom Special, but don't know when it ended.
there was also at least one secondary through train on this route.
All of these premier trains could be in the 20 car range in the winter time.
Chicago to Florida had ONE daily premier train that managed Chicago to jacksonville in slightly under 24 hours. For quite a few years it rotated between three routes, so each operated every third day. After the third train, which actuall ran the shortest route and the only one still possible end to end as it was then, the Dixie Flagler was discontinued the others ran every other day. They were the City of Miami and the South Wind. All had essentially identical end point times, but some had it easier than others.
Of these three, the one that always carried the largest passenger load was the City of Miami. It also hit the fewest large population centers between end points. It is no loger possible at all due to line abandonments and downgradings. The City's operators were Illinois Central, Central of Georgia, Atlantic Coast Line. ICRR called the shots. It was entirely painted in ICRR colors regardless of car ownership. Its route, ICRR main Chigago to Fulton KY via Effingham, Centralia, Carbondale. A couple of St Louis cars were added at Carbondale. South of Fulton it ran through Jackson, Tennessee to Birmingham on a route stitche together in 1908 by use of the original ICRR main through Jackson Tenn to Grenada Mississippi, then trackage rights on the M&O Jackson TN (Perry Switch) to Corinth MS (Ruslor Jct), new ICRR track there to Haleyville AL, then trackage rights on Southern to Jasper AL, then trackage rights on Frisco to Birmingham. From there, Central of Georgia to Columbus GA and Albany GA, then on ACL to Waycross GA, then on to Jacsonville on ACL, the FEC to Miami. This line was the fastest in possibilities of the three. North of Fulton the line was almost all double track and part had a 100 mph speed limit. So far as I know only Columbus GA to Waycross was unsignaled. Only Haleyville to Birmingham was very crooked and slow, and some of the CofG portions were not very fast. Fulton KY to Corinth MS is now an unsignaled short line. Part of the ICRR section there to Haleville has been abandoned, part of the ACL between Albany and Waycross has been abandoned.
The South Wind alternate ran Pennsylvania RR, Louisville and Nashville, ACL. It hit the most in population centers, running Chicago, Indinapolis, Louisville, Nashville, Montgomery, Waycross, then the same Jacksonville - Miami as the others. I believe Logansport - Indianpolis was unsignaled and know for sur that Indianapolis - Louisville was unsignaled. Parts of the L&N were double track up to the early 1960's, I think about 160 of 490 miles, but their maximum speed limit was 70 mph. Montgomery to Waycross was, and is, unsignaled. This is the route run by Amtrak's Floridian, most of the time. Penn Central's deterioration led to several reroutes, and the route north of Indianapolis has been abandoned, in part if not completely. This route was the longest in distance.
The Dixie Flagler, which was discontinued in about 1957 had the shortest route of the three. It ran Chacago Evansville Nashville Atlanta Waycross, then same as the others. The railroads involved were Chicago and Eastern Illinois; L&N; Nashville, Chattanooga, and St. Louis, ACL (Atlanta Birmingham and Coast at one time) This line is still all in place, all signaled, all main lines, and all now CSX. It is also heavily congested with freight traffic. It was always the most difficult of the three. The AB&C portion is relatively crooked and never had fast speed limits. The NC&St.L main is cross-grain to the Appaalachian chain, meaning lots of curves. Particularly the W&A portion between Atlanta and Chattanooga is very curvey and 3 hours for 138 miles was about the best ever time and could not be done today. Likewise, Chatanooga to Nasville, despite have more straight alignment, is relatively slow, taking 150 miles to go between points that are about 130 miles apart by road, due to going southwest out of Chattanooga for 38 miles before turning north so as to find a crossing point for the Cumberland Plateau that did not require a long tunnel, which would have make the route impossible in 1850. The crossing at Cowan, Tennessee, remains a helper grade to this day.
There were other routes operated up to the late 1950's on a two nights one day to Jacksonville basis. Both Southern and L&N operated day trains between Cincinatti and Atlanta that gathered late departure overnights from such places as Chicago, Detroit, and Cleveland. South of Atlanta these trains ran overnight to Jacksonville. Time Cincinatti to Atlanta was on the order of 12 to 14 hours, which is about as good as could be done today, if that good. Desipite gathering from multiple northern origins, these trains never carried the load in their later years of the fast three, if they ever did.
The 1920's premier Chicago to Florida train, the Dixie Flyer ran on essentially the same route as the Dixie Flagler but on a two night one day to Jacksonville schedule. It became a secondary run after the introduction of the fast three. It was down to a couple of coaches only south of Nashville by the late 1950's and was killed in about 1964 or 5.
The Illinois Central - Central of Georgia secondary train, the Seminole, ran daily on an early evening departure late morning arrival at Chicago basis with a one night and one day schedule to Jacsonville never had a lot of renown, although it did have fairly good ridership, at least north of Birmingham. I remember seeing it with about 8 coaches and 2 sleepers through Martin, Tennessee in 1962. But, again, this route is no longer possible.
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To do a reasonably fast, that is as in approaching reasonable drive time, say about a 50 mph or better average between end points over the Chacago to Jacksonville route would require a huge bundle of money, and would not generate a worthwhile patronage. Only if some of the fast train schemes were to be built that could be used by this service might it be feasible. There has been a midwest high speed scheme around for a long time, but so far it has generated nothing but studies. It would seem really reasonable to develop some moderate to high speed service between the cities in Ohio, Indiana, and Cihicago / St. Louis on one side and Buffalo / Pittsburg on the other, but there seems to be no will to do anything real. If this sort of thing happened so that the Chicago to Ohio River portion became fairly fast, then a revisit of work needed further south would be in order.