Good advice already. Just to repeat/add to a few points:
Some of the increased appreciation is already reflected in the prices. It's not that the day Metro opens the station on Wiehle, everyone will wake up and say: "Oh, wow. A new Metro stop close by! Now my property's worth 20% more!" Properties close to there have been marketed as "Metro coming soon" for years.
Having said that, when Metro does open, it will help home values in the immediate area. I bought a home closer in before a Metro station was completed. Prices had already been going up in anticipation of the stop. But once it opened, it gave prices an additional boost. I'd expect the same thing to happen there.
Still, there's more to location than a nearby Metro stop. A few years ago, I was marketing a property not far from where you're talking about--in Shadowood. It's already close to major roads, schools, etc. But people consider the entire community and the community's history. I found that the condo's price was depressed because of the past history of the complex.
But the most important piece of advice is: Buy because you want to live there. Make sure you like the townhouse, and make sure you like the location. Don't try to be a real estate speculator. It's too risky in this market. At best, assume very modest price appreciation on the property you buy. And make sure you get a good value on it--don't overpay just because someone tells you, "Oh, don't worry. Once the Metro opens, prices will skyrocket." So: Good value going in on a property you like in a location you like.
Hope that helps.