Many authorities consider the New York Central's class S-1 4-8-4s, erected by American Locomotive Company in 1945-46, to be the apex of steam locomotive design in North America. Intended for long-distance, heavy-duty passenger service over the Water Level Route, they rivaled the emerging diesel in horsepower, efficiency and availability. When in service on long-distance trains such as the Commodore Vanderbilt and even the Twentieth Century Limited, they were known to make the entire run from Harmon, New York to Chicago with only a few servicing stops, whereas older power had to be changed en route.

The 6000 number series for the Niagaras was intended to reflect their nominal horsepower. No. 6000, shown here in this builder's broadside photo, was delivered in 1945 and was designated as class S-1a. Another 25 Niagaras, Nos. 6001-6025, arrived in 1946 and were assigned to class S-1b. No. 5500, which was equipped with oscillating cam poppet valves, was also delivered in 1946 and was designated class S-2a. The Niagaras had roller bearings on all axles and also on the side rods. For specifications of the S-1 class, see commentaries on the following photo of No. 6000 and that of No. 6020.