If he was confident no one would know him on that trail he cannot have lived locally IMO. Yet he knew of the trail and Creek. Why?
This is speaking generally about the bridge and it being not well-known, or only easily known to locals. And it's all just my .02. Purdue's campus is only about 1/2 drive away. The student population ranges from local commuters to national to placing among the highest in the US for international students. It's feasible local students tell others on campus or in town about the 'cool railroad bridge', it's also feasible someone from another continent could've visited it. Word travels, kids need breaks from studies, a quick trip to see it or walk it wouldn't be outlandish. (I'm not implying BG is/was a college student, only that students from literally around the world can know of it, so if mentioned in passing or a pic posted on SM their families and friends can know, and so on.)
http://www.jconline.com/story/news/...dents-among-us-public-institutions/858036001/
Then there's the internet. Not difficult to find if one is searching for something to do, a sight to see.
From the perspective of bridge/rail enthusiasts, it's on bridge hunters site w/pics and if you look at the comments at the bottom of the link one person from 2008 gave directions to the bridge from Delphi, and the small parking area on 300N:
https://bridgehunter.com/in/carroll/high/
Then there are the rails to trails enthusiasts. Lots of walkers, runners, hikers & cyclists like these trails. It's on trail link and (see pic) among the icons on the map are the bridge (far right on map) and parking on 300N (my yellow arrow on map).
https://www.traillink.com/trail/delphi-historic-trails/
Also from GH's youtube video of the drive Libby's sister took to where the girls were dropped off it does appear far out in the country, quite remote & isolated. But if you note at the end as they approached the small parking lot on 300N they turned left to pull in, so they were approaching from the east (farmland), vs Delphi, the Hoosier Heartland Hwy 25 and I-65 to the west.
The point I'm trying to make with just a handful of illustrations is that it's quite possible a wide variety of people from a lot of different areas know about it and/or have visited.