This image has been scanned from a publication entitled Trains Album of Photographs Number 17: Locomotives of the Pennsylvania Railroad, published by Kalmbach Publishing Inc, w:Milwaukee, Wisconsin (the publishers of Trains magazine).
There is no date of publication nor of copyright given in the copy in my possession, but the content argues it is from the latter half of the 1940s. This publication does not display a copyright notice and thus under the copyright laws of the United States at the time may thus be public domain. It is possible that the copyright information is on a page missing in my copy, however; the pages are not numbered and thus a missing one is hard to detect.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.