It used to be that the Switch Blade could be out of sync with the Frog with solenoid points as the set up could be slightly mis-aligned, so the Blade could be behind or in front of the polarity change. It is less likely in more modern, none solenoid Motors. Another reason for binding the Switch blade to the Running rail, is that it avoids the possibility that a wheel could cause a short between the two, where the Switch is bound to the Frog group. {Back to back too short, thicker wheel rims} Just safer all round to avoid the possibility.
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