Microwaved hamburgers. They're not bad if they attendant takes the time to nuke them on a plate and not in the package. I think the Northeast Regional cafe menu is similar, but has a bit more variety. The hot dog is Hebrew National, which is good, but, again, attendants sometimes just nuke then in the wrapper, which is not so good.
I would not recommend going into the station at Washington to look for food. The pace is big, service at many of the eateries is slow, there's less variety of eateries than there used to be, and you risk missing the train.