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I think that the gap between trains and the number of trains per hour is different. I can believe that there are sometimes merely 30 seconds between trains, but train frequencies don't really exceed, let's say, 50 trains per hour (and this is being very generous) because of a combination of signalling constraints and the need to reverse the trains at the end.

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You're correct, but this is a mostly academic distinction on a network like Hong Kong's. The gap between trains is a much more important human metric, and it changes based on where you are in the network.

An example: you could fill as many trains as you could physically fit on a track coming from Central (the CBD) during rush hour; a train every 30 seconds or so is crucial to keeping things civil on the platform. As you get farther from Central, utilization falls until you reach a terminal station, where the time between trains can be more like 2-3 minutes without any problems. Through clever signaling you can achieve a much higher perceived throughput on the busiest stretches without actually running more trains.

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