In 1905, Pembroke had a 50’3″ turntable. Sadly it was removed in the 30’s when the passenger trains were run by a gas-electric, which was too long to fit on the turntable. So, we have no pictures of it, much less plans.
From the 1908 GTR bridges and building book, we know it was almost certainly powered by hand, and had no clearance limits. So, while a gallows turntable would look nice, it must have been a deck girder, probably similar to these ones:
http://trainweb.org/oldtimetrains/CPR_Bruce/orangeville_old_turntable.jpg (from http://trainweb.org/oldtimetrains/whatsnew_2010.htm)
http://www.umce.ca/biblio/cdem/photos/PC3-96.jpg (from http://themodelrailwayshow.com/LayoutDesign/?cat=5)
If you know someone who has plans for a 50′ turntable, who would be willing to share, I would really appreciate if you could put me in touch with them!
This discussion on ngdiscussion.net yields Jeff Ramsay’s Internet research on the topic of 50′ turntables. It includes a plan for a cast iron turntable from Sellers, an American company. The Pembroke turntable is more likely to have come from Dominion Bridge.