The page linked below (NW 1/4 of the Lancaster PA Quad)shows what I take to be the Mt. Gretna Narrow Gauge RR in the upper left-hand corner. It is labeled as a branch of the Cornwall & Lebanon RR, which it in fact was.
The map is a 1936 reprint from a 1903 survey.
It shows the portion of the Mt. Gretna NG from the junction with the C&L, past Conawego Lake to the Nat'l Guard firing range which was long gone by the time of the 1936 reprint. However, it shows no indication of the line up Governor Dick Hill, which should have been there in the 1903 survey.
This map repository is truely a great resource; thanks to Ed Lecuyer and Maptech for making it available. I wish something like it existed for the northwestern states
J