From the 1969 ARRL "The Radio Amateurs Operating Manual"

  The amateur distress call, QRRR, grew from the purpose of the first 
organized amateur emergency nets.  They were set up in cities along the 
Pennsylvania Railroad to aid the "Pennsy" (and later other railroads) 
with train communications in the event of failure of the railroad 
telegraph landlines - which were frequent.  The signal QRR came to be 
used to indicate that the calling station had railroad traffic related 
to some emergency.  ARRL eventually adopted this call for use by any 
amateur who had distress traffic and later the call was changed to QRRR 
because of a conflict in definitions with the international Q signal QRR.

  One of the first distress calls was CQD, coined by the Marconi Company 
about 1904 from the "general call" CQ and the letter D for "distress."  
The main problem with CQD was that it was supposed to be used only by 
ships which subscribed to the Marconi radio system and ships of one 
system were discouraged from communicating with ships or shore stations 
of other, competing, companies.  The problem got so bad that it was taken 
up in the international radio conference in 1906 where a new universal 
distress call was proposed.

  The American delegation suggested the letters NC which were already 
recognized in the International Signal Code for Visual Signaling. The 
German delegation proposed its own SOE which was already in use on German 
ships as a general inquiry signal similar to CQ (which was then used only 
by the Marconi system).  The British delegation, of course, wanted to 
stick to the Marconi signal CQD.

  The convention found SOE acceptable except that the final E could easily 
be lost in QRN so the letter S was substituted, making it SOS.  The 
convention decided that SOS should be sent as a single code character with 
a sound unlike any other character, thus arresting the attention of anyone 
hearing it.  So SOS was officially adopted, but CQD remained in use for 
some years, particularly aboard British ships.

  It wasn't until 1912, after the Titanic disaster, that SOS became 
universal and the use of CQD gradually disappeared. Titanic radio operator 
Jack Phillips sent both CQD and SOS to be sure that there couldn't 
possibly be any misunderstanding.

     -Louise Ramsey Moreau   WB6BBO/W3WRE