Many authorities consider the New York Central's class S 4-8-4s, erected by American Locomotive Company in 1945-46, to be the apex of steam locomotive design in North America. For a discussion of the merits of these locomotives, which the Central styled "Niagara" instead of the common designation of Northern type, see the commentary on No. 6000 on the following page. While the other 26 Niagaras of class S-1 had the Baker valve gear, No. 5500 featured the oscillating cam poppet valve gear, operating similarly to the valves in an automotive engine, and was classed as S-2a. It was delivered in 1945 and proved even more efficient and powerful than the other Niagaras. However, difficulties in maintaining the poppet valves led to No. 5500's being the first of the Niagaras to be retired, in 1951. Therefore, photographs of it in service are not common; this image, for which no information is available, was purchased from an eBay vendor.