When there is a breakdown or a disabled car or another problem with the Metrorail system, it creates a logjam of trains, because it is difficult or impossible to reroute the trains that are behind the incident. Once the issue is resolved, the trains continue to follow each other one by one, because of the limitations of having just two tracks.
The first train, though is the slowest train. It immediately becomes crowded with a crush load of passengers and then encounters overcrowded platforms. People take longer to board and deboard; each stop becomes a two-minute drill to deboard and board about 20 or 30 people. In many cases the vast majority of people on the platform can't even get on and have to wait one or two or three trains to finally board. It can take a couple of hours for all the trains to move through the system and return service to normal.
I have coined the term "Buffet-Line Effect" to describe this phenomenom. This comes from my observation that in a restaurant buffet line, the line moves not at the speed of the average diner, but at the speed of the s-l-o-w-e-s-t diner. Everyone backs up behind the one person who is carefully scrutinizing each and every offering. Relief finally comes when they move off to the dessert table and the whole line speeds up.
The simple and easy solution to the Buffet Line Effect is to express the trains after a delay. There are miles of empty track in front of the first train (and lots of filled platforms). If the trains that are backed up are expressed to fill in the empty space, then normal service can return within minutes rather than hours.
This will, of course, inconvenience some passengers who may have their stations skipped and either have to deboard and get on a later train or backtrack to their station. However, I believe that the overall savings for passengers will be much, much larger than the inconveniences. Given the capacity stresses Metro is experiencing on a regular basis, simple, no-cost solutions like these need to be part of their short-term strategy.
To do this well, though, requires two things: intelligent dispatching and excellent communications with passengers. Let's try an example:
Assume a breakdown at Gallery Place on the Red Line to Shady Grove. A mechanical breakdown causes a 20-minute delay and a train is taken out of service. 
There are now 5-6 trains backed up behind, probably out past Union Station. The track is empty from Gallery Place out to past Friendship Heights, and the platforms are filled to overflowing in the downtown area.
First train: Skips Metro Center and stops at Farragut North (which has a center platform). The dispatcher has been alert and makes sure a train going the other way arrives immediately or is waiting for this train. Riders for Metro Center can choose to wait at Gallery Place or backtrack from Farragut North. Train then expresses to Bethesda. [NOTE: The train operator clearly and constantly communicates to his/her passengers throughout this whole process, allowing everyone to make decisions about how they are going to deal with it.] Train immediately arrives or is waiting at the platform in Bethesda for passengers to backtrack.
Second Train: Services Metro Center (at the same time the first train is servicing Farragut North) and then expresses to Van Ness (with a center platform). Similarly, the dispatcher makes sure there is a reverse train ready to take passengers back.
Third Train: Services Judiciary Square (while the first two are servicing MC and FN). Skips Gallery Place and Metro Center and services Farragut North like the first train. Dispatcher makes an informed decision about skipping another stop to create breathing room behind or allowing train to proceed as usual. If this train is going to create its own "buffet line effect," then it should express ahead to allow the faster trains behind to proceed normally.
Fourth train: Services Gallery Place and Metro Center and proceeds normally from there, possibly skipping Farragut North if the third train skipped DuPont and created space (and also to get the riders at DuPont, who have now watched three trains go by).
So how about those riders at DuPont who watched three trains go by? In the Metro Universe we inhabit now, most of them would have had to wait for two or three trains just to board. And those trains would have been progressing at "buffet-line effect" speed. In the end, they probably reach their destination just as fast. The overall system is returned to close to normal service within fifteen minutes, with trains more evenly spaced along the tracks rather than bunched together behind a dysfunctional, overcrowded train. 
It makes absolutely no sense to have 8 miles of empty track followed by a "buffet line" bunch of six trains clunking along at half speed.
Obviously, the exact way to deal with any delay will vary every time, based on how long the delay is, time of day and other variables. This is why it requires intelligent dispatching and on-the-spot decision making to maximize the value.
The other requirement, in my opinion, is courage. If Metro were to put this into effect, they need to do so consistently for a solid six months or longer, to allow riders to get used to it and to realize that they are actually getting better service than the current dysfunctional way of dealing with delays. They have to have the courage to stand up for the vast multitudes who save five or seven minutes against the vocal few who might lose fifteen or twenty on a single occasion and then complain. I don't know if Metro has the courage to do that.
