I would guess they were used, at the very least, in Chalk Creek canyon, and at the Palisades. Go to the library site Chris Walker links to in his post above, and blow it up as large as you can get, then look at the rail joints a rail length behind the coach.
Look at the Jas. Wiseman photo on PP 166 - 167 of Dow Helmer's Historic Alpine Tunnel. Looks like, again, we have squared joints instead of staggered joints, and Fisher joiners at both places.
A John W. Maxwell photo identified as being of original DSP&P track is reproduced on P. 130 of the Memorial Edition of M. C. Poor's Denver South Park & Pacific. The photo illustrates the Fisher joints in place in the track, and a loose example is laid between the rails for comparison. the caption is vague enough so it really isn't easy to say where this was located, or when Maxwell took the photo.
Anyway, it seems there are a few images out there showing this unusual rail fastening in use. .
Cheers,
Charlie
CVM
-30-