Nice stuff! Here in Western Maryland, USA, we have a long, abandoned rail line that has been converted for use of hikers and bicyclists, the tunnels are few and far between, because our terrain is old Appalachian, worn down over the ages and nowhere near as rugged as Japan.
Interesting video! In the near of the north tunnel mouth from Yamanaka (in Google Maps it is Yamanaka, in your video Nakayama?) tunnel is a second tunnel, but it has no second tunnel mouth, it ends in rock. Was this tunnel only a service/shunting track from the station? And this thing for two tracks in 10:46, was it only for shunting too, if two trains encounter on the single-track railway?