Many people were aware of the bridge before the murders, but had never seen it in person. Railroad and industrial artifact fans from far and wide knew about it. It's the 2nd tallest bridge here in Indiana, and just happens to be a very old railroad trestle/viaduct which has withstood time, and I wouldn't doubt there have been efforts to have it torn down.
I've driven right around the area in question, the bridge is not easy to find just driving by there. The spot where the girls were dropped off is easy to miss, even driving at 35 mph on C.R. 300. Considering the latest PC, it appears that LE believe the vehicle seen at the old CPS building is involved in the case. So, the perp/BG would have known about the trail head by the Freedom Bridge over IN 25, and would have known that if he parked at the abandoned building, he could walk a short distance, get on the main trail, and could walk a ways and be at the bridge. Why didn't he park on the other side of IN 25? We all know the answer. Everything about BG's movements make sense to me, the shortest distance between two points is a straight line. Each section of his path(s) that day scream "efficient", to me. Quick, no wasted time or energy. No reason to waste time walking longer distances, unless he was going out of his way to conceal himself, perhaps after the murders.
Not only do I believe he knew where the bridge is located, he'd timed every part of his plan/fantasy/what have you. I would venture to guess BG timed the walk from the trail head to the bridge, the walk across the 850 foot bridge, and the walk from the SE end to the creek, across from the CS, which is a short distance covered in a matter of several minutes, according to YouTube videos I've seen. The distance from the SE end to the CS is much shorter than the length of the bridge. The distance from the slight bend in the trail, to the NW end of the bridge, is maybe 800 feet. So BG had to A) Get the girls off and away from the SE end of the bridge, and B) Down to the creek, as quickly as possible. That way, line-of-sight, nobody would have seen the group in the gorge, even one person or a group if they were on the trail.
JMO
-FD