They already run a free shuttle bus to the BWI Rail Station. It's not that far at all. Actually, something I ran into last night (don't know where) mentioned plans to run a "monorail" (that was the word they used) to the rail station from the terminal. I don't know if that's still in the works or not. The station is really just across the road and south a bit, no farther away than some of the satellite parking.

Having some sort of rail link to the station might cut the time to get over there vs the shuttle bus, but that isn't the real weak link in this chain anyway. The weak link right now in getting to DC is the MARC train service, which at many times of day is only once an hour down to Union Station and doesn't run on weekends or holidays at all.

A bit of thought put into the MARC stuff would probably result in a much more useful link without building anything new. Example: why not have dedicated trains that run an express BWI-Union Station back and forth? Those trains don't need to continue on to Penn Station in Baltimore; people going to Balt from the airport can ride the light rail. People going between DC and Balt or points between can ride the standard MARC service that continues to run the entire length and all stops of the Penn Line (which currently serves the airport). Heck, if they just took one extra train and ran it back and forth from Union Station to BWI, they should be able to double the frequency when combined with the existing schedule. Then all they have to do is run some service on weekends. That's probably the touchiest one, but if they think of it as service from BWI to DC rather than a commuter line, I think they might be able to cover the costs of running some trains on the weekends.