What is the difference between Texas Eagle 21 and 321? The times are the same from Chicago to St. Louis but 321 shows coach seats available while 21 indicates coach sold out.
I could be wrong but I think "321" represents the car that is taken off at St. Louis - very frequently they add a car to the 22 and remove a car from the 21.
As a side note, I took the 421 (service thru to LA) over Thanksgiving weekend and was amazed at how many people had booked a roomette just for Chicago to St. Louis. When we arrived in St Louis, most of the sleeper cleared out.
The only difference between 21, 321 and 421 is the car of the train. 321 is the car that goes from Chicago to St Louis only, 421 is the 1 coach & 1 sleeper that goes to LA and 21 is the cars that go to San Antonio. (22/322/422 is the opposite.)
If going between stations served by 2 or more of these "trains", chose the least expensive one. Of course being combined, they run on the same schedule.