Wikipedia "History of the Pennsylvania Railroad" .

Chris Baer's "A General Chronology of Pennsylvania Railroad"  via the PRRT&HS website.

Chris Baer's "Corporate Names of the PRR Family"  via the PRRT&HS website.

"World's Busiest Mountain Railroad" by David P. Morgan. Trains magazine, April 1957. Courtesy of Kalmbach Publishing. 

1936, March 25, Coverage of the 1936 Flood. Courtesy of Jerry Britton. (Scan pending.)

Brochure: The Pennsylvania Railroad: Serving You Through Progress in Transportation. Courtesy of Jerry Britton. (Scan pending.)

Northern Central Railroad: A History, by Robert Williams. Courtesy of Jerry Britton. (Scan pending.)

Packet of Misc. Correspondence. Courtesy of Jerry Britton. (Scan pending.)

1949: Correspondence on proposed purchase of property from Commonwealth of Pennsylvania for industrial development. Former Harrisburg Steel Company plant. Courtesy of Jerry Britton. (Scan pending.)

1955, Announcing the New Organization of the Pennsylvania Railroad. Courtesy of Jerry Britton. (Scan pending.)

1949, April 8 Terminal Routes of Maryland, Philadelphia, Philadelphia Terminal and New York Divisions. Courtesy of Frederric Abendschein.

An index and some issues of The Pennsy, the PRR's employee magazine, are available in "Keystone Crossings" Books & Periodicals section.

An article on the PRR's Manual Block System is offered by its author, Al Buchan.

Brochure: The Pennsylvania Railroad: Serving You Through Progress in Transportation. Courtesy of Jerry Britton. (Scan pending.)