The Builder does not use a fixed number for the transdorm (or didn't as of a year ago, anyway - I'll squint and try to see the number card next time I see it go by in whitefish.)
In a short consist, the full sleeper is 0730 and the transdorm is 0731.
In a long consist, the full sleepers are 0730 and 0731 and the transdorm is 0732.
If you have a roomette reservation in the 31 car, look at the room number. Normally they sell only transdorm rooms 17-24 to passengers (and make you board through the 30 car and walk forward on the upper level.) If you have a number 1-8 you are almost certainly in a full sleeper, and if 9-14, certainly in a full sleeper.
That's part of the problem here with the yield management system. The available number of rooms jumps from 5 bedrooms and 14 roomettes, to 5 rooms and 22 roomettes (transdorm open), and only then to 10 bedrooms and 28 roomettes (2nd sleeper added, transdorm closed) or 10 bedrooms and 36 roomettes. If the yield management system works when the transdorm is closed, it is out of whack when it is open, and vice versa.