IAD to DCA is kind of a pain but doable. There's the silver line express bus that leaves IAD every 15 min and takes you to the last stop of the silver line. Then you have to take that into the city and change trains to the line that goes to DCA (can't remember the stops or lines but the DC metro is really easy). All in all it takes a minimum of 1.5 hours based on timing of trains etc. There is also the 5a bus that takes you from IAD directly to either Rosslyn or L'Enant Plaza metro stations where you can catch the metro to DCA. This route is a few dollars more and can be quicker depending on time of day and year. Sometimes the 5a only runs once per hour so if you just miss it, the silver line express is faster, but timed right the 5A is faster. The other issue is early mornings and late nights. Don't have it memorized but during the one month I was there I had to Uber once because the metro wasn't running yet.... As far as BWI, there is no good way that I know of to get there. Im sure there's a way between different buses but you're probably looking at hours. I had a commuter flight divert to BWI and ended up getting a one way car rental for $25 and it still took me an hour without any traffic. I only had to do it once so someone may know a better way. I'm not on the bus so I don't know how many trips are out there.