It is the type of walkway that shows up on PTX's earlier cars:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=ZPg4AQAAMAAJ&pg=GBS.RA1-PA849

See also the pair of Ohio Cities tank cars on the bottom of p.135 in Steve Hile's UTLX book.

David Thompson