Nancy and Dennis looked at the recent mailcar post http://justacarguy.blogspot.com/2013/01/way-back-when-us-post-office-was.html and commented about this very action you see above you,
the mail was transferred when the train did not stop in a town.
when the train came through it would leave a bag on the first one and pick the bag off the second one.
I vividly recall going with my Grandfather to the train station in Canton, NY in the early 50's: he would hang a mail bag on one of the posts, then the train would come through without slowing down, and BLAM - BLAM! The bag he hung would be gone and another bag would be swinging on the other hook!
Photos from http://www.hagopgaragem.com/