Yes Dovecote, you need to be sure that you book the correct train number; not a car number. You want the Train 421 sleeper. The 2130 car is a train #21 sleeper and will dump your friend in the San Antonio station for the night.
Unfortunately I don't have an answer for you... and excusing my ignorance, but why do you perceive this criticality [assuming there is something that I don't know to be learned here]?
gmushial,
If Dovecote were to book train #21, then the person would have to get off the train in San Antonio and board train #1 later that night to continue their trip west. If they book train #421, then they can remain in their seat/bed fast asleep and the car is cut off the Eagle and attached to the Sunset when it arrives.