David Alpert created the timeline above showing the evolution of the Metrorail system, right up to the opening of the Silver Line’s first phase. You can read his posting about it on his blog, Greater Greater Washington.

I had to look back to remember that when I arrived in Washington in 1988, the Red Line ended at Silver Spring and the Blue Line at Addison Road. There was no Green Line.

Readers often write in to ask where Metro is going next. Their letters usually are accompanied by a suggested route. Some would like to have a Metro line that tracks the Capital Beltway around the region. Others want to extend the Orange Line west along Interstate 66 at least to Centreville, or push the Blue Line south to Woodbridge. I’ve heard from Marylanders who wonder when Metrorail will link Baltimore and Washington, or at least extend down from Branch Avenue into Southern Maryland, to take the commuting pressure off Routes 4, 5 and 210. I’d like to see Metro take advantage of the two lanes on the Woodrow Wilson Bridge being held in reserve for transit.

But I think the answer is, “None of the above” — at least, no time soon.

Two problems: Passengers and money.

With Metro out of the construction business after completing the original system, it’s likely that local jurisdictions will need to put together the financing and building plans for new lines, or line extensions, just as they have with much of the Silver Line. They will be looking at that $5.6 billion price tag for the Silver Line and the anger among the Dulles Toll Road drivers who have to support the project. The politicians will be wondering, “Why do I have to be so brave as to take on responsibility for some project when I might not be around for the ribbon cutting?”

But there’s also a practical problem with most of the potential projects we’re thinking about: Most of the communities through which those rail lines would pass don’t have the population densities to support an urban subway system. At the service frequency such a system requires, there would be a lot of near-empty trains traveling opposite the rush-hour direction. That’s why planners are thinking more about light rail, rapid bus systems and commuter rail when they think about transit for these various corridors.

You will see this issue arise as we continue discussing Virginia’s plans to ease congestion in the I-66 corridor between the Beltway and Haymarket.

For its part, Metro will be looking at various projects to ease crowding and increase capacity within the core of the Metrorail system.

Meanwhile, you might enjoy these other graphics on the Silver Line: