I am thinking of getting a monthly Washington DC - Baltimore pass. Essentially all North-East Corridor trains are now reserved, but many of them still accept monthly passes.

Question: can I just get on board a reserved train with a monthly pass? Or do I first have to stop at the Amtrak ticket counter, show the pass, and get them to issue a (no additional cost) ticket? I called up Amtrak, and the agent thought it was the latter; if true, given the queues at Amtrak counters, that would be enough to kill the deal.