The Oslo subway system is entirely in Norwegian, which is odd since most of the major signs in the rest of Oslo are in both Norwegian and English. However, I was able to ride the system every day for a week, all over the city, and out into suburbs without any problem and without knowing any Norwegian. You just look at your map, see where you want to go, and get off when you see a sign with the same combination of letters. You don't need to understand what the words mean and once you take a few trips the whole system pretty much begins to make sense in any language. Seems to me the same would apply to the DC Metro system.
Far as Norwegian and English go, both are grounded in Old West Gothic. A mere thouand years ago they were pretty much the same language.