Zoo Tower gets its name from the zoo that sets between it and the river. It controls the wye where the east/west mainline across Pennsylvania meets the Northeast Corridor. The junction is complicated enough that it is still staffed. That is, no one has figured out how to control it remotely. I normally don't follow postings in PA, but a picture of an interior that is still in good shape is rare. Especially of one that is so complicated. Mike Brotzman has a lot more pictures and information.