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When running in the underground portions of the muni metro system, the trains are controlled by computer and because of this, they stop at the same place along the platform every time. Unlike bart, which is similarly controlled by computer, the spots where the doors will open are not marked out. Even without these markings it is still possible to tell where one will need to board. As people enter and leave the trains, their feet wipe clean the dirt form the very edge of the train platform leaving clean marks at each door opening. By observing and counting off the marks in sets of four it is possible to tell where to stand to enter at a particular door on a particular train.

The current debate about surveillance turns out not to be a feature of September 11th, instead being born of the move to mobile telephony and digital switching that started in the 1980's. govexec.com has a thorough article that gives the history of the debate here. It is well worth a read if you have the time.

Classic science short about the scale of the universe.