No, really, they do. New York's Metropolitan Transit Authority (something like Transport for London) has produced an outstanding video that shows why making some subway trains late makes others less late:
Yes, the idea is that sometimes delaying a train can prevent further delays by not compounding the gap between trains. Anyone who has waited impatiently on a hot subway platform might find this concept counterintuitive, but transportation experts generally agree that that the evenness of service is as crucial as avoiding individual delays.The MTA video makes a compelling case. The key insight is that once a platform gets crowded enough, due to constant feed of new passengers and a delayed train, it becomes slower for the next train to debark and embark passengers. So an already delayed train gets more delayed as it progresses down the line. The solution? Spot a train that's getting near the critical delay time and give it priority to progress through the network even if this involves delaying other (less delayed trains).
It's a great example that, even in what we regard as relatively simple systems, there can be a complex interplay between entities that produce highly unintuitive results. Deliberately delaying trains can actually be good for the system as a whole (if not for the passengers sitting in the delayed train with their faces pressed into a fellow passenger's unwashed armpit).