Railway Sorting Offices. 
The offices were locations where mail was sorted for railway movement. Many of these were located at railway junctions to provide the most expeditious service, but not all. Some were located in strategic locations where mail and railway facilities were positioned to meet train and ship schedules. These offices fed not only the TRAVELING POST OFFICES, but also BAG TENDERS – unmanned railcars that moved bagged mail between main centers. Some sorting offices had unique cancels.