[greglindahl]

People in that station actually talk to each other, so a few % of app users in the crowd might result in a large number of people knowing.

I'd hate to imagine what would happen during peak if a train came in on an unexpected track AND a different train on the expected track - it is not unusual for multiple trains to be announced at once. Then you'd have a pile of people trying to go up those narrow stairs while a pile of people were trying to go down.