[ARC5] What is a "radio range filter"?

[joldenburg2 at new.rr.com]

It appears that 25hz is still in use in industrial app's in the Niaagra Falls area:

http://electrical-science.blogspot.com/2009/12/history-of-power-frequency.html

Jon AB9H
---- Glen Zook via ARC5 <arc5 at mailman.qth.net> wrote: 

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Methinks a typo!

Virtually every part of the United States uses 60 Hz power! Glen, K9STH Website: http://k9sth.net

      From: D C _Mac_ Macdonald <k2gkk at hotmail.com>
 To: Richard Knoppow <1oldlens1 at ix.netcom.com> 
Cc: ARC-5 Mail List <arc5 at mailman.qth.net>
 Sent: Tuesday, April 26, 2016 3:17 PM
 Subject: Re: [ARC5] What is a "radio range filter"?
   
I never heard of any US area that used 60 Hz power but areas around Niagara Falls used 25 Hz as late as 1950s!  Mac, K2GKK/5