I priced a roomette one time on the Crescent from Atlanta to N.O. which has low use of rooms since it is daylight and the train ends in N.O. Then I tried to purchase 2 rooms. Price was higher. Went back and try to buy one. Price was higher. Apparently, buying the first caused both to be higher when purchasing them on one reservation.
I also found that if you use Amtrak site, selecting a room seems to "lock" that room for a while at the price listed but times out if you don't purchase. One one browser, I locked a room then immediately went to second browser and priced a room. It had gone up. I exited both browsers without completing a reservation and then tried again to get a room a bit later. Price had gone down. Note my test was done late night and there were likely few if any others booking rooms at the time.
If you are purchasing two rooms, price one and if it is low, grab it. Then buy the second on a different reservation. Thus if price goes back down later to what you paid for the first, you can call and get a refund of the difference.
