I have mentioned this before but since the topic out here is about listing annoying self-made "rules" by OBS, I would point out my Southwest Chief journey last November where the coach attendant was adamant about not letting me sit at a window seat at an empty row in a half-empty train because "all seat pairs are for families" and single riders should all sit next to each other in one half of the coach. He enforced this "rule" all the way up to Albuquerque in spite of the three coaches being around 50% empty at all points of time during the journey.
That IS the company guideline when assigning seats on a long haul train. That's why there is a lounge car, so you can get the your window access. Only 50 percent of the seats are "window" seats, and given a choice, about 95 percent of passengers would request one (or require one, if such a thing were allowed). The passenger manifests that are given to OBS employees do NOT generally indicate how passengers are grouped (unless it's LITERALLY a group, and indicated as such on the manifest), so it is necessary to hold block of seats for families and small groups. Do you think it would be appropriate to separate a parent traveling with three small children from them?
There IS one way you can be guaranteed a window seat, however: book a sleeper.