What is the symbol for a queue?

I don't know if this is canonical but, does this count?

Also here are examples of queues in SDL-88

http://www.sdl-forum.org/sdl88tutorial/4.ProcessCommunication/4.1_Signal_input_queue.htm

I don't have a source to cite, unfortunately, but I recall seeing it represented as an isosceles trapezoid.

Actually I found an example of it here (though maybe not the most authoritative of sources).

Edit: From comments it looks like the example site I had linked is no longer available. This is a recreation of what was in the original document: