Insulators -- A Clinker ?

[nw-mailing-list at nwhs.org]

In a message dated 12/18/2004 10:09:19 P.M. Eastern Standard Time,  
nw-mailing-list at nwhs.org writes:

So, how did the N&W mark the DS circuit in the pre-telephone days of  
single-wire Morse Telegraph circuits?
 



Is this an obscure reference to note that train crews had no method of  
communicating
between attended stations BEFORE telephone service ?
                                                            Harry Bundy
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