I've been going through all my SR&RL books looking at whistles in the photos, and I'm quite sure at this point that it's not from No. 15 or any of the other "big" engines. (Bob Werner may have been an authority on SR&RL motive power, but I just can't see what he was thinking here.) However, there are some photos of No. 7 (P&R 'Calvin Putnam') with a whistle similar to this, even down to the pointy "acorn" nut on top. So, if it really is from an SR&RL engine, then my best guess would be No. 7.