Image by MW Transit Photos used with permission.

We recently got this Metro question in our inbox:

Hi GGWash, I have a question about timed transfers. When I lived in the Bay Area and used BART, the system there had timed transfers at stations. At certain stops, you could walk across the platform to a waiting train on a different line. Neither train would leave until riders had the opportunity to transfer.

Obviously you can't walk across a platform in the majority of metro stations, but why can't Metro arrange timed transfers at places like Metro Center/L'Enfant/Gallery Place/ Fort Totten. Particularly on weekends when service may be every 20 minutes?

Two weekends ago I ran from a Red Line train to try to catch a Blue/Orange/Silver train only to watch it pull away as I reached the bottom of the escalator. The next train was 15-20 minutes away. It was infuriating.

Thanks for the question! There are a variety of reasons why Metro doesn't do this. I'll break them all down, but first let's understand…

Why BART has timed transfers

Timed transfers on BART are easy to implement because the system was built for it. Several stations are designed for cross-platform transfers between trains going in the same direction, which allows two trains to stay at the platform while people cross between them relatively quickly. There are extra tracks at the MacArthur, 19th Street/Oakland, and 12th Street/Oakland City Center stations to allow two northbound trains (at any of the stations) or two southbound trains (at MacArthur) to stop at the same time with an island platform between them.

However, BART is relatively unusual among rapid transit systems in having stations designed in this way.

Timed transfers were part of the system's original plan in part because it has four lines from the east side of San Francisco Bay merging onto one pair of tracks running across the Bay into downtown San Francisco, where the system's busiest stations are.

BART routes (thick grey lines are planned routes). Image by BART.

In order to achieve higher frequencies on some of the East Bay lines, there is another line, the Richmond-Warm Springs line, that only runs on the east side of the Bay and does not serve downtown San Francisco. On evenings and Sundays, this line is the only service to Richmond and Warm Springs, so passengers from these termini need to transfer to get downtown. Timely transfers were more important at these chokepoints.

Even when cross-platform transfers exist, timing them may not be worthwhile

Since the majority of trips on most rapid transit systems are to or from the main downtown core, systems are not generally designed to require passengers to transfer to get downtown. Not having four lines share the same track pair allows higher frequencies, which make timed transfers less necessary.

Most rapid transit systems aren't really designed around transfers for this reason, so it makes sense that few of them are set up to allow cross-platform transfers, which are particularly tricky.

However, one other American rapid transit system is notable for having a large number of them. The New York subway has four tracks on most of its downtown trunk lines: two for express trains, and two for locals. At stations served by expresses, there are usually two island platforms—one between the local and express tracks in each direction—so passengers can easily transfer between them.

2/3 platform at Fulton Street by Harrison Leong licensed under Creative Commons.

However, there are not timed transfers between local and express trains in New York. Implementing them would be too difficult at the high frequencies the subway usually operates and would actually slow trains down too much. Late at night, when frequencies are lower, express trains generally run as locals, so timed transfers aren't needed. Trains are occasionally held for a few seconds to allow a cross-platform transfer with another train that is about to arrive, but this isn't done during rush hour and it's only practical because cross-platform transfers take so little time.

Metro doesn't have express tracks, and they would not have been a good investment on a system of our size, so cross-platform local-express transfers like New York's aren't an option. Metro's transfer stations are between lines crossing perpendicularly, so building cross-platform transfers would require significant divergence from direct routes.

Cross-platform transfers do exist Metro at stations where two lines diverge and there is an island platform: Stadium-Armory, East Falls Church, and King Street. However, since there is very low ridership between the ends of the lines that diverge at these points, a timed transfer to, for example, let Vienna-bound Orange Line passengers switch to a Largo-bound Blue Line train at Stadium Armory would be of limited benefit.

Timed transfers at transfer stations would take a long time

Metro's main transfer stations are not built for cross-platform transfers like certain BART stations are, which means that timed transfers would require trains to wait at stations long enough for passengers to transfer in both directions. For safety and liability reasons, the waits would have to be long enough that even relatively slow-walking passengers starting in the cars furthest from the escalators could make transfers without running.

An eight-car Metro train is 183 meters long, so at L'Enfant Plaza and Metro Center, transferring passengers would have to be able to walk half this distance—about 91 meters—while transfering. At Gallery Place-Chinatown and Fort Totten, they would have to be able to walk a full 183 meters, since the escalators for transfers are at the far end of one platform.

Chinatown station by SounderBruce licensed under Creative Commons.

With a relatively fast walking speed of three miles per hour, and budgeting 25-30 seconds to walk from the platform to the escalator and get to the other platform, trains would still need to pause for about a minute and a half at L'Enfant Plaza and Metro Center, and for three minutes at Gallery Place. At Fort Totten, the wait might need to be three and a half minutes.

Such long waits would be a serious inconvenience to passengers continuing past transfer stations. It would also increase Metro's expenses, since each train would take several minutes longer to complete each run.

Timed transfers would be hard to schedule

If Metro were to schedule timed transfers at the major transfer stations, it would be a fairly complex task to make the trains line up.

Optimally, one would want to have four trains present during a timed transfer: one on each of the four tracks at the transfer station. That way, a transfer would be available for passengers going in each direction on both of the lines. However, since WMATA has three downtown transfer stations, it would need to time trains to arrange such transfers at each station.

For example, a Red Line train heading toward Shady Grove would need to meet northbound and southbound Yellow or Green Line trains and a Silver Spring-bound Red Line train at Gallery Place-Chinatown. It would then need to meet another Silver Spring-bound Red Line train at Metro Center, along with Orange, Blue, or Silver Line trains in each direction.

Meanwhile, the Yellow or Green Line trains would have to meet up with the Orange, Blue or Silver Line trains at L'Enfant Plaza. And, to complicate things, the travel times between each of the three station pairs are significantly different.

Making this work would require relatively high frequencies unless trains were to wait significantly longer at downtown transfer stations, and it would only be able to benefit a minority of transferring passengers since, for example, a given Red Line train can have a timed transfer with the Green or Yellow line, but not both.

Metro needs to run reasonable frequencies, making timed transfers unnecessary

Fortunately, there's a much simpler solution than trying to introduce timed transfers in a system that isn't well-designed for them. Metro simply needs to run trains at reasonable frequencies, even at night and on weekends.

Timed transfers make the most sense on systems with low frequencies and long waits between vehicles, such as commuter rail and suburban and small-town bus routes serving low-density areas. They also work best when it's possible to have a single simultanious transfer point for many lines, such as a bus terminal. In such situations, the delays for passengers travelling past the transfer point can be a worthwhile price to pay for the benefit of allowing short transfer times between half-hourly or hourly bus routes.

However, an urban rapid transit system like Metro should not be running headways longer than 12 to 15 minutes, even on weekends and evenings. At these sorts of frequencies, the benefits of timed transfers aren't worth the sorts of delays that Metro would have to introduce into its system to make them possible.

Do you have a question? We regularly post a question to the Greater Greater Washington contributors and post appropriate parts of the discussion. You can suggest questions by emailing ask@ggwash.org. Questions about factual topics are most likely to be chosen.

DW Rowlands is a human geography grad student and Prince George’s County native, currently living in College Park. More of their writing on transportation-related and other topics can be found on their website. They also write on DC transportation and demographic issues for the DC Policy Center, where they are a Fellow. In their spare time, they volunteer for Prince George’s Advocates for Community-Based Transit. However, the views expressed here are their own.