I am doing the opposite. I booked a flight into DCA, but out of BWI. I will be at the US Capitol building on my last day, which is a Thursday. What time do I need to leave the Capitol to make it to BWI for a 5:51pm flight on CO. From reading the posts I would prefer to take the MARC train.
Is there a Metro stop near the Capitol that can take me to Union Station? I will need to get to BWI in time to check a bag before I go through security. (btw is there an Elite Access security line at BWI? or something equivalent?)

If I just took a taxi, would the extra cost save me much time at that time of day?

thanks