Way To Go USPS !

Russ, could you clarify what is meant by Chicago M. Western tube station?

With the pneumatic mail the letters were placed in capsules and sent through tubes to the postal station that handled the delivery address. The letter was either dispatched from or received at the M(id)Western Tube Station which placed the cancel on the cover. The tube stations were normally in the basement of the post office since the tube were normally underground. This is the first tube station cancel that I can recall seeing.
Edit: the picture showing the "roceteers" was taken in a New York City tube station.

Still digging for information, didn't find what I wanted but I did find this rather good and funny article

http://curiousexpeditions.org/?p=321

Here is a quote from it I found rather funny. " On at least one occasion the tubes carried not just mail, but a live cat."