A quick look at the map inside the cover of "Narrow Gauge to the Redwoods" shows the NPC running along the Russian River from Duncans Mills to Markham (about halfway to Jenner). At Markham the line spits, one route going north to Laton (quite a ways inland from Jenner), the other crossing the river and running southeast to Willow Creek. Unfortunately, the map doesn't show any private lines in the vicinity. There's a photo on page 164 of a locomotive that ran on a 5'5" gauge track of iron-faced wooden rails from Duncans Mills up Austin Creek Canyon, although this doesn't sound like the same location you're referring to. The previous post actually sounds more promising. Also, I recall hearing of a horse-powered line that ran from the south bank of the Russian River to Duncan's Landing on the coast. I suspect it might be worthwhile to check old copies of the "Western Railroader."