Artist's mock-up of the Demonstrator after becoming the first of the Atlantic Coast Line's Z-1 Class. Today it survives at the America Rails Museum.
The Pennsylvania Railroad's lone S1 Duplex was not exactly a smash hit. But Baldwin learned from its mistakes and this would allow the ACL Z-1 and the PRR's own T1s to have more success.
When the PRR first revealed the S1 Class 6-4-4-6 #6100 in 1938, many people were skeptical. On one hand, it was fast and impressive. But OTOH, not only did the presence of the tired-and-true K4 4-6-2s and R3 4-8-4s make it pointless, but it was also too long to be operated east of Crestline, Ohio. As such, Baldwin would turn to other railroads when testing the idea of Duplexes for a while. To that end, they build what was essentially a scaled down version of the S1 with a 4-4-4-4 wheel arrangement. This locomotive also featured a centipede tender, Walschaerts valve gear, roller bearing rods, a full-width diaphragm cab, and was streamlined in a design created by Otto Khuler.
The engine was first tested on the Pennsylvania Railroad's mainline to Chicago. While the engine was impressive, the Pennsy still saw little point in getting more at the time due to the K4s and R3s suiting their needs fine. As such, Baldwin went to dest the Demonstrator on several other railroads. The first one to try the engine was the Baltimore & Ohio Railroad, who tested it over Sand Patch Grade and their mainline in the Midwest. However, the engine tended to slip on the steeper lines in the east of the network. The next railroad to actually consider the design was the New York, New Haven, & Hartford, who ran the Demonstrator on several passenger services normally hauled by the I-5 Hudsons. However, its a bit too big and the New Haven did not order any. The final railroad to consider the type was the Atlantic Coast Line, who ran it over their mainline from Richmond to Jacksonville, FL.
The ACL was satisfied with the design as it had improved the balance of the running gear through the use of smaller, lighter parts, reduced bearing loads, and improved steam distribution. However, it did increase the complexity of the locomotive and the duplexes tended to be "slippery", requiring more careful handling than conventional steam locomotives. Nonetheless, once the ACL began training engineers on specifically how to operate the engine, they were proven to be decent machines with enough punch to them. These machines would become legends and classified the Z-1s.