Bout a Rail Europe Pass. When making seat reservations, which I find to be a royal pain, since it's a separate website, How in the world does one ensure you are not on the second deck? It randomly assigns.
Where are you trying to book seat reservations? In France, a Eurail pass (which I assume you mean) can be a bit of a faff.
Yes- it is so challenging. Seems like 3 different websites, and they will not link. . Trying to book reservations from paris to Bordeaux.
Official SNCF website:
https://www.sncf-connect.com/en-en
In 1st class, you should be able to choose a seat and the deck from a seating chart.
In second class, you are assigned seats. The upper deck has higher number seats.