This is listed as a rail-trail (which it is), but it's not a typical such! The Dunderberg Spiral Railway (DSR) was to be a gravity railroad, with two very steep up-grades that hauled the cars to the top (where there was to be a resort), and ten miles of gently sloping down-grades. The Timp-Torne Trail initially follows a short bit of near the end of the DSR down-grade, to the unfinished cut-and-cover tunnel. Then it follows the steep section for a little ways. The Ramapo-Dunderberg Trail (red dot on white rectangle) continues up the hill on the DSR. The TT (blue rectangle) cuts to the left (south).
You'll continue to climb, cutting across the hillside to make the slope a little more gentle. You'll recognize that you've come back to the DSR when the trail becomes nearly level and curves only gently. This continues to the hard-rock tunnel (only about half dug). Do stop and check it out! The TT trail continues around to the 180 degree turn-around curve and then the other side of the tunnel.
The TT trail continues on. My interest was in the DSR, not the TT, so I left the TT and walked on a construction road built for the DSR, to rejoin the DSR higher on the hillside. Then I walked down the hill (steeply!) on the up-grade. It's about 90% complete, with some gaps where the down-grade crossings were to be built.
The RD trail is also worth hiking if you're interested in seeing more of the DSR. It has another partially-constructed tunnel. Only about 60% of the DSR was graded, and none of the buildings were begun.