The Nevada County Narrow Gauge interchanged their cars with the Kitts Railroad along Greenhorn Creek. The Kitts railroad was known to have its own locomotive and would use NCNG flatcars to haul logs from the woods down to one of the mills along the NCNG.
Higher up in the Sierras the Towle Bros and later Reed Lumber operations had atleast 2 interchanging roads, both of which had strap iron rails and were horse drawn with their own flatcars. The body of one of those flatcars is reported to still exist deep in the canyons.
And of course who could forget the Tonopah boom in Nevada and the interchange between the Tonopah RR and the Carson and Colorado which lasted a brief few years.
-=Andrew=-