Between 1909 and 1923 the Chicago & North Western received 186 Pacific types (4-6-2) of class E from the American Locomotive Company. The entire class was converted from Baker or Young to Walschaerts valve gear sometime after their delivery, and in the 1940s the C&NW undertook an extensive rebuilding program in which most of these engines were rebuilt to class E-S. Details of the rebuilding, and which modifications were applied to each locomotive, have proved difficult to sort out; however, it appears that most of the class were fitted with a stoker, ALCo power reverse gear, cross-compound air pump, and train control. These engines had 75-inch drivers and cylinder dimensions of 25x28 inches; for other specifications consult the commentary on No. 1667 following.

In 1941 Nos. 1617 and 1620 had their boiler pressure raised to 200 p.s.i. and were fitted with a streamlined casing, raising their locomotive weight to around 271,000 pounds, for service on the Minnesota 400. In 1952 they were renumbered by dropping the first digit, becoming 617 and 620. Despite losing that assignment to diesels they kept their shrouds until retirement, No. 1620 being scrapped in 1956. She is shown here at Elroy, Wisconsin on May 26, 1951, in a photo acquired via eBay attributed to Paul W. Prescott of Conneaut, Ohio.