The first group of the K-5b subclass of Pacifics, represented here by No. 4907 (originally CCC&StL 6507), came from American Locomotive Company in 1926. They had 79-inch drivers, cylinder dimensions of 25x28 inches, and a boiler pressure of 205 pounds per square inch; for other specifications, consult the commentary for No. 4933 following. Two K-5b sisters, Nos. 4915 and 4917, received streamlined shrouds and disc drivers in 1936, as seen in a Gary Overfield photo in George Elwood's collection. As a result they weighed more than the other K-5s, at 317,000 pounds. They powered the Detroit-Cleveland Mercury, later the Cincinnati-Chicago James Whitcomb Riley.

This image of class K-5a Pacific No. 4907 was provided from the David Loeffler collection. The engine is apparently being towed backward from the passenger terminal to the roundhouse by Hudson No. 5395 (note the Walschaerts valve gear in reverse position and the flagman on the pilot protecting the rear of the movement). The probable location is Cincinnati, where New York Central (specifically, Big Four) locomotives could end their runs at a major terminal having arrived from two different directions. The date has to be the late 1930s, after No. 4907 assumed that number in 1936 but before the introduction of the Central's Gothic lettering in 1940. To further pin down the date, No. 4907's flat font end suggests that her smokebox has received the "Selkirk" nozzle-and-stack modifications for improved gas flow through the boiler tubes; this change was applied to many NYC locomotives after being worked out in the design of the J-3a Hudsons of 1937. The photographer remains anonymous.