Surprised that no one has mentioned Minneapolis - Des Moines - Kansas City - Dallas - Houston / San Antonio
best with conection with the Southwest Chief at Kansas City, then you can have Chicago to Texas and Twin Cities to California through service by some switching.
Now reschedule toe Eagle to an early am arrival late pm departure at Dallas, have a Houston section that combines at Longview, throw on some through cars to Memphis out of Little Rock, then extend as a day train to Chicago, which is already there north of Carbondale, and you have increased the service possibilities far beyond the increase in train miles. Oh yeah, this train should continue directly west from Dallas to El Paso, maybe even Los Angeles. You could also continue east of Memephis on the old route of the Tennessean, except daytime to east Tennessee, all the way to Washington or New York. This might work to combine with teh Crescent, but maybe not. That combination should occr at Lynchburg, not Charlottesville, by the way.
I think that the Lakeshore Limited route could support about three daily trains spaced out, and you could split at least one at Cleveland to go Columbus-Cincinatti and another to go Indinapolis-St. Louis-Kansas City.
I am not forgetting the need for midwest to Florida already mentioned by others. But as has been noted, there is considerable expense required to make a reasonable and realiable schedule possible. In the 50's early 60's, the City of Miami / South wind left Chicago around 9:00 am arrived Chicago aroudn 6:00 pm and took about 23 hours to Jacksonville. They were around 30 to 32 hours Chicago to Miami.
The problems there:
City of Miami route: no longer possible. several sections abandoned others now branch line status.
South Wind route: different Chicago to Indianapolis. major work needed Indianapolis to Louisville and quite of bit of work also needed Montgomery to Waycross.
Dixie Flagler Route: all still in place, all still main line. Heavily congested with freight. Even double tracking throughout might not be enough to ensure reliability. This was alwasy the tightest schedule of the three due to a very curvey route through Georgia nad parts of Middle Tennessee. No part south of Evansville ever had over a 70 mph limit and most south of Nashville was 60 mph, curves permitting.
George