An 8 car express train during rush hour that will only run from the line's first stop but will stop at it's last high traffic/ downtown stop to reduce crowds in cars.
Rush hour Metro cars can be packed tighter than a can of sardines...not a pleasant feeling in the early hours of a Monday morning on the way to work. But after it passes its last transfer station, the conductor drags a bunch of empty cars down to the end of the line while the rest of central DC is trying to fit in to a car so they can be at work on time. Each line should have an Express train that runs from its starting station, but make its final stop at the line's last transfer station. This will control passenger traffic because those travelling to the end of the line won't board and will save space for those travelling within the downtown. The Express cars can expand to 8 car trains, and the full lines can actually cut down on a train or two, since after its last transfer stop there will only be a handful of passengers. It's cost effective in that there's no need to add another conductor because the trains will run as before, it will only be stopping earlier. And because the high traffic trains won't be travelling so far, they can save time but turning around sooner, and will have more trains available passing through high traffic stations (and there might not even be a need to add/subtract cars from trains from the high turnover)