The SP&S had three RPO routes on the system.  Most people think of the mainline from Portland to Spokane, but the Astoria line and Oregon Trunk each had one as well.  The "Spokane, Pasco & Portland" RPO operated on the mainline, the "Portland & Seaside" RPO operated on the A-line and the "Fallbridge & Culver" RPO operated on the Trunk.. 

     Other postally related items are perfins.   These are stamps that were purchased by the company for use in mailing company business.  The stamp is punched with the initials of the company in a descending diagonal pattern.   Many large companies across the country used this practice to discourage employees from pilfering postage for their personal use.