h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The Orange Line does run out to Tysons Corner, at least not yet. There is still some debate as to whether the extension should be above ground (approved by all federal, state, and local governments) or below ground (the preferred choice of most residents and businesses, especially businesses, but also the most expensive and potentially time delaying).

In addition there's been talk about a "Silver Line" which would run out to Dulles. Interestingly enough, this was envisioned many many many years ago, but like all public works projects in America, people poo-pooed the costs at the time and now everyone realizes it's way more expensive now and even more necessary, but they still poo-poo the costs.

Also, several stations on the Orange and Blue lines have colored signs that tell which train is coming next, not just at Pentagon. I don't think I've ever seen them work, though. Not that it matters as there is a thing that tells you the approximate time of the next train, the color, and the direction on the platform.

Also the Yellow Line goes above the Potomac, not under. The Orange and Blue Lines go under the river between Foggy Bottom and Roslyn.

And yes, for the most part it is a useless line, but for those of us who want to get back to the Alexandria area and aren't keen on taking the meandering Blue line to get there (it roughly goes west first and then turns eastward), it does have it's usefullness.