The last couple of times I have ridden the Three Rivers east out of Chicago, they have started boarding at 10:10 so that everything is set to go for an on time departure. Lately, they have even been putting the mail/express cars on in the station before the train leaves, so it can head straight out of town with no stops in the yard. It always leaves from the run-through track #28 on the east side of the station. I think that for sleeping car passengers, the lounge sometimes closes a bit earlier than 10, and they just have you wait in the priority boarding area and board before the coach passengers do.