Have I mentioned they will build elevated rail service to Dulles Airport? This sounds quaint and European but there is already a dedicated, virtually traffic-free road to that airport, in addition to three or four totally usable back routes.
[W]ith the single exception of BART in the San Francisco area, every U.S. transit system actually reduced social welfare. Moreover, they [Winston and co-authors] could not identify an efficient pricing policy or physical restructuring of the rail network that would enhance any system's social desirability without effectively eliminating its service.
Bottom line: If the Tyson's metro extension passed the cost-benefit test, it would raise real estate prices so much that it could be funded out of property tax revenue. But without tons of external subsidies, metro expansion wouldn't stand a chance.
P.S. I'm baffled by comments that call Tyson's "ugly." It's Beverly Hills in Virginia, as far as I'm concerned. But then again, I was baffled when Alex Tabarrok told me he missed Oakland, California. That's what I call "ugly." When a big developer builds an area from scratch, it wants to maximize real estate values by making residents happy. What's the result? Clean, convenient, corporate living, Big Box stores, and McMansions - not Oakland.