When the Texas Eagle (421) and the Sunset Limited (1) combine in San Antonio, they are one train but continue to use their separate numbers because Amtrak's system can't handle an alternative. The crazy part comes going the other way so if you go from Alpine to Houston, you'll be offered two choices, Train 1 (you stay in your car because it goes to Houston as the Sunset Limited) or 421/1 (it looks like you have to take two trains but you have to exit your car and reboard (but not right way) a car going to Houston on the Sunset or you'll end up heading to Chicago on the Eagle.

The result of all this confusion is two sets of rates. So San Antonio to Alpine or back allows you to pick either train but one may be cheaper than the other (sometimes considerably) because on Amtrak bedrooms, roomettes, family rooms, etc are all priced independently of each other and the same goes for the "two" trains. You may find a bedroom or family room on 1 cheaper than a roomette on 421 or roomettes differing in price depending on demand on the particular train number. If few people in rooms are going east from the west on the Sunset and a lot are going North on the Eagle to Chicago, then the rooms between Alpine and San Antonio in 421 may be full of people headed north but not on the Sunset in 1 continuing East. Since most people dislike waking up in the middle of then night when not getting off in San Antonio, most will choose the one train option. For those from the west getting off in San Antonio or heading west from there, they get a good deal as they can pick either train. This is also true if you were going from Alpine west as the section called 421 may be fuller with through passengers from north of San Antonio but the section from NOL called the Sunset may not.