The Southern Railway’s work with the Washington Southern predates the RF&P as the Southern had its own line to Alexandria, VA, interchanged freight with the PRR (PB&W) and B&O and used Washington Southern to get its passenger trains into DC before Washington Union Station was constructed.

There are multiple files on the WS, Potomac Yard and the RF&P, here (Box 83 File 3859 - 175 scanned items) is a two page letter from AJ Cassatt of the PRR to Sam Spencer written 2-21-1900 describing the concept of the RF&P.