As my canopy photos have shown (here and here) the canopy in the survey area in Brunei was not particularly dense hence the proliferation of ground cover and hence the term, jungle.
We wondered where this jungle railway went when we stumbled across it while laying out our survey grid. It just seemed to start in the middle of the jungle.
After the surveyor cut his line, a 'bridging crew' would follow along behind and cut smaller trees to form a walkway, or bridge. Typically three poles wide and nailed together, it made walking the lines considerably easier for the rest of us and in most cases kept our feet dry also. It also meant that relatively few people were actually tromping across the jungle floor.
Actually, the shot below is of a pretty shoddy job. If we'd done it properly, as we did most of the time, there wouldn't have been any blow-out. Here, a large part of the energy of the charge is moving upwards as the shot hole blows out and the noise generated when the dirt falls back to the ground also degrades the signal. But when a shot goes well, there's nothing to photograph.