In signalled territory, the timetable and the operating rules often stipulate a minimum time interval between trains following each other on the same track.

Recent plans by Naperville historians to build a memorial to the April 25, 1946, rear-end collision involving the Advance Exposition Flyer and the Exposition Flyer illustrate the value of the rule.  The trains departed Chicago Union Station simultaneously, with the Advance preceding the Flyer on the center track, under protection of automatic block signals.  A reprint of a contemporary story indicates that a trainman on the Advance was preparing to comply with the first two paragraphs of Rule 99.  The engineer of the Flyer might have been passing restrictive signals at track speed, a forbidden practice that train crews routinely engage in to keep time.  As long as the train ahead keeps moving, nobody is the wiser.  Should the train ahead stop, however, the result can be a broken train and 47 dead people.

This accident is not usually mentioned as one of the proximate causes of subsequent Interstate Commerce Commission mandates establishing maximum speeds on tracks protected by various levels of signalling, from none to cab signals with automatic train stop or train control.

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