Well, I went to DC as planned and sure enough, as reported by the many good people on this thread, there was indeed plenty of baggage room on both trips. It didn't really end up being a trip where there was much time for riding anyway, so it wasn't a big deal except for the rude, lying misinformed people who answer the official AMTRAK phones.
One of the highlights of the trip was on our walk back from the C and O: we saw three mudcovered riders go by in Georgetown, two MTBs with trailers, one with pannier