Salida Barrel Transfer

I’m trying to find out how the Barrel Transfer at Salida functions. I’ve got only one picture (The Story of the C&Ts, Antoinette Gibbons, p. 6) that shows 4 towers and the bridge the gondolas were turned on. The bridge was rolling on a inclined level and was turned by cables from the lower towers. That is what I could discover from the photo.
What were the to higher towers for? (looks like there were counterweights to turn the bridge back) and why were they connected to each other? How were the cars fixed while turning? Were the cars moved by locomotives or by cable engines?
Hope someone can answer my questions and thanks in advance.
Jens