Thanks for your kind comments on the book.
I am just as perplexed as you as to why the triangle was installed, and I have spent about forty years trying to work out a logical reason. Your suggestion that at least at some stage the preferred locomotive orientation was opposite north of the triangle compared to south has occurred to me too. I think that is quite a likely explanation in the early years when they handling a lot of traffic and using two locomotives. At that time they were not working north of the triangle but east.
Unfortunately the triangle fell out of use before 1917 and I was not able to find anyone to interview who had first hand knowledge of how it was used.
I cannot give any precise estimate of the length of the apex of the triangle, it might have been long enough to take a complete train, but I am doubtful of that. It definitely dead-ended within the confines of the paddock, that was my observation, and several people who saw the earthworks confirm that.
No, I do not know of any photographs of the triangle, if there were they certainly would have gone into the book.