I should add that Washington Union Station is sort-of a "mixed bag" of platforms. Tracks 7-19 (used by MARC commuter trains and most Amtrak trains that originate/terminate in Washington, including the Acela Express) are high-level platforms that are accessed at the same level as the main concourse, just like Boston South Station. But the difference there is that one does not feel the elements at WAS, mainly because there is a buffer zone, an area between the initial gate you enter, and where the bumping posts are by the beginning of the platforms.
Tracks 20 and above (used by Superliners, trains to or from points south and west, and VRE commuter trains) are the low-level platforms, and these run underneath the station so you have to take an escalator or elevator down to track level. Even so, here too there is a buffer from the elements with all the corridors one has to pass through.
I would think there are some people who might try to sleep in the station, particularly those waiting for Train 190, which leaves WAS at 3 AM, and Trains 170 & 2150, which departs 5 AM. Even with ticket offices closed, everyone travelling should be able to produce a ticket since there are Quick-Track machines right in the waiting area.