They do this with a few trains in a row until there are enough people waiting in the station to fill up a train to some acceptable level
The "acceptable level" probably doesn't account for everyone getting a seat on the continuation train. This used to happen with buses in Wuhan 10 years ago. If two buses on the same route met up and each had only seated passengers, one would send its passengers to the other bus to stand for the rest of the journey. This was annoying because I would travel places at off-peak times just so I could get a seat.
And of course, because this happens occasionally, it affects everyone using the Shanghai Metro because they all need to build in contingency time when planning a trip to get somewhere by a certain time for an appointment.
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The "acceptable level" probably doesn't account for everyone getting a seat on the continuation train. This used to happen with buses in Wuhan 10 years ago. If two buses on the same route met up and each had only seated passengers, one would send its passengers to the other bus to stand for the rest of the journey. This was annoying because I would travel places at off-peak times just so I could get a seat.
And of course, because this happens occasionally, it affects everyone using the Shanghai Metro because they all need to build in contingency time when planning a trip to get somewhere by a certain time for an appointment.