You could can't book the long-distance trains from Philadelphia to Washington, but you can from Philadelphia to Alexandria. Is that a long enough trip?
Years ago, I would book space from NYP to ALX on the Crescent or the "Silver" trains and simply get off in WAS. The rail fare was the same. This 'secret' enabled me to enjoy much roomier (and reserved), seating, less 'hubbub' of passengers on and off (stopped only to receive), use of diner and lounge cars, etc. The only downside was the train took about a half-hour longer than the corridor trains to make the trip due to longer station dwell times.