Metro will soon place an order for a new series of railcars. The new cars, the 7000 series, will be quite different from Metro's current fleet of cars.
One notable difference with the cars is that they will be quad-sets. They will still be married pairs, like WMATA's current rolling stock, but instead of having a cab at each end of the pair, a cab will only be present at one end.
A second married pair facing the opposite direction will give the set of four a cab at each end. By eliminating cabs in half the cars, this configuration will give the cars more passenger capacity. A control panel will be located at the other end of each pair to facilitate movement in yards.
Image from WMATA