As Ryan Avent says, Metro expansion is great, especially when it’s done properly, but you can’t just keep adding new branches in the suburbs:
One final point: Metro is a network. When a new extension is built, the additional connectivity increases the value of all the other nodes on the system. But while that increase in value is significant, it’s not nearly as great as the benefit conferred on people located along the extension, who suddenly have easy access to the whole of the system. And meanwhile, the usage generated by the extension does generate some direct and indirect costs on other users.
These costs are increasingly borne by users in the core of the system, where growth in the number of trains and passengers have led to crowded conditions on platforms and back-ups during peak periods. To some extent, this can be addressed by increasing peak fares, but given the obvious value of Metro, the growth in the system’s spokes, and the fact that the District is better suited than almost anywhere else in the metro area to handle increased density, it seems clear that new core capacity is needed (as well as a new river crossing over or under the Potomac).
Probably the best way to add capacity would be to construct the proposed “separated blue line” through downtown. Among the other virtues of that plan, it would have an immediate and obvious benefit to many people in Virginia, as well as improving the performance of the overall system, thus broadening its potential constituency. The Brown Line idea shown here also has a lot of merit and would, I believe, be less hideously expensive. But either of these proposals would cost a lot of money—there’s no cheap way to build heavy rail beneath an existing city. That said, the benefits would be enormous, both to the areas directly served and in terms of the enhanced value to the rest of the network. The completion of the Green Line has already had a completely transformational effect on swathes of the city.