Engine Whistle Signals

Hey, How many of you know your train whistles signals?

I see that back then four long whistles was the signal to call in the flag regardless of direction. In later years it was changed to 4 whistles to call in the flag from the west (i.e., to an eastbound train) and 5 to call in from the east (for a westbound train). I wonder what accident resulted in that change ...