From what I can tell, or was told by someone who may have been way back when hehe, look at International Morse Code. The "Q" is "_ _ . _". Apparently, when ever the Queen of England went on a boat trip, the captains would blow the horn like that so the people knew who was coming, and the draw-bridges were opened quick.
Cross the pond, and we look upon our locos with same reverence that a queen demands, and just like one, steamers wait on no one. So we do indeed have a Royal Family living here in our country, permanently. And we treat them as such, demanding that everyone get out of the way with the same notification, "_ _ . _".
Believe it or not, in doing a little digging on this subject I found a discussion forum, The Horn and Whistle Forum, dealing with the use of these by railroad trains as well as other vehicles, such as steamships, etc. Find it here, http://www.hornwhistleboard.com/.