Seems like nobody's talking about coach seat assignments. I never rode coach on the Texas Eagle, but I rode coach on the Capitol Limited and the Crescent last June. On the Capitol, the conductor on the platform directed us to a specific coach car depending on our destination. However the train wasn't very full due to a missed connection from a western train in Chicago, so on board, the conductor just told people to find seats wherever they liked. The Crescent was pretty full, with only 2 coaches, and the coach attendant assigned actual seats on the platform, depending on your destination. I think he was trying to put passengers traveling overnight into one car that they could have to themselves without being disturbed by people getting on and off in the middle of the night. As there was only one coach attendant this slowed boarding at every station.
I think the short answer is that if the train is crowded, they'll assign seats (they also want to put groups, especially families with children, together.) If the train isn't so crowded, they may just have you board a particular car. And you never know what it will be until you get to the platform and the train comes in.
