Matt Yglesias has another in a series of posts making the same point: "Nobody Will Take Mass Transit To Work Unless You Build Tall Buildings Near The Stations."
I agree, which is why I suggest that one prerequsiste for rail expansion, whether it's high-speed or plain old commuter lines, should be zoning laws allowing greater density near stations. For example, I don't know if there will ever be a demand for more high-rises in Fall River or New Bedford, Massachusetts, but there's no way that South Coast rail makes sense unless those cities promise not to stand in the way of vertical development. Same thing goes for the extension of the Green Line into Medford.
Photo of Moscow from Our Surprising World.