Tunnel of the Dead

There is a forest on the other side of the valley that doesn't seem to have any paths, so today Bruno and I set out to explore it. We found the old railway track which led eventually to this closed up tunnel. I'm pretty certain a species of vampiric beasts live in here, coming out at night to feast on the locals. 
I tried to find a path over the top of the tunnel, but it became very narrow and then disappeared altogether. There was a steep 300 foot drop the other side which Bruno and I found ourselves sliding down at one point, so I abandoned and turned back.