It's debatable whether Metro will be able to handle another few tens of thousands of daily commuters going through the downtown core to get to Crystal City. The system's already crammed up at rush hour and nobody has the budget for capital improvements to start to put more tracks through Gallery Place, Metro Center, and L'Enfant Plaza.
It would be interesting though, if Amazon was prepared to invest in capital improvements for Metro...
Judging from Seattle's experience, Amazon is not really prepared to do anything other than garner support from the business community and support a token streetcar. And Metro's capital needs are probably much bigger than Seattle's and are politically more complicated due to three jurisdictions that are frenemies in the best of times.
It's worth noting that Seattle and DC have vastly different representations on their boards. In DC this is two representatives picked by the governments of DC, VA, MD, and the feds each, and in some cases not even appointed by elected officials. In Seattle, this is a board consisting almost entirely of elected officials of counties and municipalities.
Seattle's method works because the responsibility of elected officials is quite clear and their voting records are transparent, whereas a diffuse chain of responsibility via appointment is not.
Metro rideship is down from its peak, overall. The system handles over 600,000 rides per day now; it handled around 750,000 per day between 2008 and 2012.
Riders commuting from downtown DC to Crystal City would be going against the prevailing flow into the city. Plenty of room on those trains in the morning.
Riders in MD would have a hard time, but a person would have to be pretty dumb to move to MD for a job in Crystal City.
Because Metro's reputation suffered when operations couldn't keep up with rising ridership. What will prevent Metro's operations from suffering again when an influx of Amazon employees stresses the system again? Would it be better if those workers got onto the already-jammed freeways?
> against the prevailing flow
Only if they board the trains between the downtown core and Crystal City. Odds are, if they're boarding from the northwest or from newly gentrified neighborhoods, they need to pass through the downtown core to get to Crystal City, which puts them on the same trains as people commuting in from the suburbs. They may get on pretty close to where the suburban commuters get off, but there's still an overlap. Not to mention the overcrowding on the downtown core platforms themselves. Keep in mind as well that Crystal City-bound trains from the downtown core stop at the Pentagon first, which has its own non-negligible share of commuters - those trains may not be full, but they sure aren't empty.
It would be interesting though, if Amazon was prepared to invest in capital improvements for Metro...