DC blocks bikes from the metro on those occasions...notably the 4th of July and Inauguration Day. At 9am going out of town on the 4th of July, they wouldn't let me on. Had to go home, drop off the bike, and come back to get on a nearly empty train. Kind of irritating, but in general, the trains are going to be jam packed, and they could fit another 3-4 people on there without a bike in the way. While it would be more reasonable to still allow access to certain points or at certain times, it complicates what is already a very complicated day for the transit system.