A NEC ride can be part of a trip to DC, too.

There are 3 major airports serving the DC area: Dulles, National (officially "Reagan"), and Baltimore-Washington (BWI). Dulles is way out in the VA countryside with (for now; construction is finally underway on a Metro line) mostly taxi and bus connections to town; National has a DC Metro station at the terminal; and BWI has the Amtrak BWI station on the terminal bus loop.

While National is the easiest to use for transit, it has limits due to a relatively short runway, restricted air and ground space, and gates mostly owned by certain big airlines. So tickets are expensive and service has odd limits except to the large hubs. Congresscritters love it because it's almost downtown, but they don't have much incentive to minimize travel cost - and some of those odd flights are probably for their benefit.

Dulles is The Big Airport - everybody serves it, though "budget" airlines like Southwest have somewhat limited service. But the trip to town by taxi is a budget buster, and the buses too unless you take a (mostly for employees) shuttle to a Metro stop in VA.

BWI is interesting because Amtrak (schedule - PDF) is convenient to the airport, not expensive for the Regional trains (Acela is more expensive and some don't stop there), and even on the Regionals (which run up to 100 mph or so) it's maybe 1/2 hour from DC Union Station. Buy your ticket before leaving home, print it at home or a local Quik-Trak machine, and then just walk right up to the platform when you get there (dodge the lines at the machines there). And all the budget airlines serve it heavily. With a little care in making reservations, you can get to BWI with at most one stop from most places Southwest flies.