Just doing some searches online, I found this image of a Rock Island train coming out the east portal of this tunnel dated 1975.
As it is copyrighted, I can't post it here, but I can copy the link.
http://www.railpictures.net/viewphoto.php?id=372817&nseq=9
After looking at the posted bridgehunter photos first, seeing what this washed out, overgrown and rusty line looks like today, and then lookin at a fully operational, polished rail line with loaded train on it back in the day from the same angle really puts in perspective what 32 years of neglect can do.
This is Tunnel 1 (of 4) on the old CRI&P line.