Getting across town from G'town to Union Station can take a while in the mornings, but there's MARC commuter train service northbound (see MTA Maryland's website), and light rail, bus, and subway once you get to Baltimore. On the whole, I'd rather be spending the weekend in DC than in Charm City, so if you can swing it, I'd try it for a month or two. If you're planning to share her rent, you can find more affordable housing in B'more, so you'll have to factor in the trade-off of commuting costs (both financial and opportunity costs) against the benefits living in DC.