It all has to do with maintaining an even gap between trains. Otherwise, a small delay will swell into a large "service gap" between the slow train and its predecessor. The gap increases because the lagging trains must linger longer and longer at each platform to let the growing crowd of impatient commuters board. Pausing the train in front to let the slow train catch up helps maintain an equal spacing between vehicles and keep the system running smoothly. This same principle can prevent traffic jams between automobiles. As Kate Baggaley suggested in the April 2014 issue of Popular Science, "Pay attention to the car behind you, and maintain an equal distance between it and the car ahead. With a buffer, if the driver in front briefly brakes, you won't pass the hiccup along."