There is no policy news as such today, so I thought I would delve into an ultra-weedsy topic — WMATA’s network design and how to improve it without breaking the bank.

Right now Metro is in the midst of a pronounced ridership decline that seems driven primarily by a declining level of service frequency, which in turn is driven by the urgent need to do a bunch of repairs, which in turn was driven by a series of bad breakdowns that themselves reduced ridership. Let's operate on the assumption that the SafeTrack initiative will end someday and be successful. Ridership should then rebound, since the DC metro area has only gotten bigger.

And here we have a problem. Metro, like a lot of transit systems, portrays itself in terms of “lines” — Red, Yellow, Green, Blue, Orange, and Silver — that depict service patterns. But in physical infrastructure terms, the picture is a bit different. What we really have is three trunk lines that run through the core of the city. One of those trunk lines doesn’t branch on either side, so it precisely corresponds with the Red Line on the service map.

Another trunk line carries the Yellow and Green lines through the city, doesn’t branch in the North, but in the South does branch into a Yellow Line serving South Arlington and a Green Line. The third trunk line historically carried the Orange and Blue Lines and branched in a conventional way east of Stadium-Armory. West of Rosslyn it also branched, with the Orange Line running through North Arlington and out to Fairfax, but with the Blue Line running down to merge with the Yellow Line.

That worked well enough, but then WMATA added a new branch extending from East Falls Church out to Tysons Corner and then on (when construction is complete) out to Dulles Airport. This branch, dubbed the Silver Line, then shares tracks with the Orange Line in Virginia, shares the Blue/Orange trunk line through the city, and shares tracks with the Blue Line east of Stadium Armory.

The problem here is that having added a new branch without adding any new capacity to the core, every Silver Line train that runs through downtown needs to displace either a Blue Line train or an Orange Line train because there’s no more room in the core tunnel. Displacing Blue Line trains is undesirable because that’s the only commuting option for Van Dorn Street and Franconia-Springfield. But displacing Orange Line trains is undesirable because those Orange Line stations have a ton of passengers, and ideally the Silver Line service should have been adding capacity to that route rather than being *merely* an extension.

WMATA’s official proposal for alleviating this capacity crunch is to build a ton of new tunnels, as seen in this core loop idea.

This loop derives from older proposals for a separated Blue Line that was supposed to add service to Georgetown while alleviating the core crunch. It evolves into a loop because burgeoning development near the Waterfront and Navy Yard stations are starting to stress the Green Line’s capacity, so the loop addresses that as well.

When you look at this map, it shows a lot of expensive tunnel digging that accomplishes very little in terms of expanding Metro’s geographical footprint. It also relies on a lot of awkward transfers to achieve its core capacity goals.

So here’s another idea I got from the great transit writer Alon Levy.

In phase one, you turn the Blue Line into a rump shuttle service between Rosslyn and the Pentagon, and serve Franconia-Springfield exclusively as a branch of the Yellow Line. This would annoy a bunch of Blue Line riders (they were annoyed when WMATA did something similar and called it “Rush Plus”). But it costs $0 and it expands the system’s total capacity by using the existing infrastructure more efficiently. The value proposition just can’t be beaten.

Then for new construction, instead of a new east-west tunnel through the core, you take advantage of the fact that there’s an existing rail tunnel connecting L’Enfant Plaza to Union Station. You do the necessary construction to make it possible to send Yellow Line trains through that basic route, and then send the Yellow Line off north of Union Station along the tracks that are currently the Eastern half of the Red Line.

Last, you get the big new construction project. A subway tunnel leading from Union Station under H Street and Benning Road would carry Red Line service that then merges with the Blue Line at the Benning Road station and proceeds East. Silver Line service would simply follow the Orange Line.

This plan addresses the core crunch and the Green Line crunch, is easier to build than the loop (no river crossings, less density along the route), and expands Metro’s footprint by a lot while also improving service for some lower-income areas in Ward 7 and Prince George's County.

The main downside: You need to admit that the H Street streetcar is useless, and thus the new H Street tunnel is not redundant. That’s easy for me to say, because it is totally useless. But it would be embarrassing for the city to launch a big new construction project whose premise is that the last big construction project was a mistake. That said, it’s hard to make things better if you won’t admit to mistakes.

A potential extra project would be for Virginia to make something out of the rump Blue Line by constructing a subway running from the Pentagon out to Annandale under Columbia Pike and then eventually build a circumferential route under Route 7 connecting the Silver Line at Tysons to the Blue Line at Bailey’s Crossroads, and then to the Yellow Line at King Street in Alexandria.

All that extra suburban building might end up requiring even more core capacity, and that would be the time to build the separated Blue Line hopping from Rosslyn to Georgetown and then east across M Street.

This is an abbreviated web version of The Weeds newsletter, a limited-run policy newsletter from Vox’s Matt Yglesias. Sign up to get the full Weeds newsletter in your inbox, plus more charts, tweets, and email-only content.