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some good info here.
Also FYI "January 7, 1929 The New York Central Railroad's "20th Century Limited" runs a record seven identical sections. "
Regarding scheduling "Where ABS signals were in place, the second train could leave as soon as the first train got far enough down that line that the signal would clear for the second train, followed by the third, etc. Where there were no signals to provide rear end protection for the train ahead, then the 20 minute rule typically applied. This meant the following trains had to wait until 20 minutes had elapsed after the departure of the train ahead before they could follow. This gave time for the train ahead to send out flagmen in case something delayed it."
So depending or railroad and length of signal blocks determined the timing gaps.