Sort of makes sense now. I didn't even notice that until now. I assumed the tunnel was associated with the road visible in GE. Thanks for pointing out the proper location of the tunnel.
Anyhow, I'm still not certain which route the NG line took. The section of NG track which went through the tunnel shown in the GE image you posted above seems to spawn from the alignment of the SG overpass. As mentioned, it seems to be inconsistent with the curve and gradient chart. But then, these aren't necessarily 100% accurate.
I'm still confused as to which route the NG took in the 1950s between the sideroad at the far left of the image I posted, and the original narrow gauge tunnel, which fortunately has now been identified.
I assume the overpass was constructed around the beginning of the SG era? Was there a road deviation built around this time? If so, did the original NG line just run parallel to the road on the right?