Welcome to the Hyde Park-Leechburg walking bridge. At 600 feet long, it is one of the longest foot bridges in the United States. It connects not only these two boroughs, but also the two counties of Westmoreland and Armstrong.
The existing piers once supported a wooden railroad bridge dating back to 1886. The original railroad bridge was damaged twice by major flooding events. A wood plank footbridge was built in its place in 1920. The current steel deck bridge, designed by the John A. Roebling's Sons Co., opened in 1955.
For more information, an excellent article on the bridge and its history can be found here:
http://old.post-gazette.com/lifestyle/20030914herep2.asp
You are looking for a small container hidden in the usual manner. Its no longer a wooden bridge, right? You do NOT need to climb or lean to retrieve this cache. Stealth is not usually a concern here, but be mindful of other bridge walkers.