Yes, more train tunnels! (Those that have done Mountain History know what I mean)
This is the South end of the Zayante Tunnel that runs through the small town of Zayante. This tunnel was originally 220 feet long and has since been converted into a records storage facility.
(UPDATE: 2/5/22)
The recommended approach has now been marked with a waypoint nd the hide has been moved slightly as it seems this area has activity now. A spoiler pic has also been uploaded to go along with the hint.
Looking at the facility now, you'd have a hard time telling it was ever a train tunnel!