For the increasingly heavier express trains between New York City and Chicago, the New York Central ordered a locomotive from ALCO in 1945 with a 4-8-4 wheel arrangement, which was supposed to deliver a permanent output of 6,000 hp. Paul W. Kiefer was responsible for formulating the requirements. After S-1a No. 6000, 25 S-1b were built, which were not referred to as “Northerns” by the NYC, but as “Niagaras”. They easily exceeded the required power with up to 6,680 indicated horsepower and were among the most efficient steam locomotives of all time.

The boiler was made as large as the loading gauge allowed. For this purpose, a steam dome was omitted and the stack was made as low as possible. According to the state of the art, roller bearings were used on all axles and on the connecting and coupling rods. The seven-axle tenders had an exceptionally large coal capacity of 46 short tons and comparatively little water. This can be explained by the large number of water troughs in the NYC network, which allowed water to be replenished during the journey.

The 6000 still had drivers with a diameter of 75 inches to maximize pulling power. On the S-1b this was increased to 79 inches and to compensate the boiler pressure was increased to 290 psi. When the pressure was reduced back to 275 psi, the cylinder diameter was increased by half an inch to compensate again. In 1946 the S-2a No. 5500 was built, which had Caprotti valve gear and was also called “Super Niagara” due to its increased efficiency. However, the S-1 were not rebuilt due to the more complicated maintenance.

The Niagaras were on duty six days a week. On the seventh day, workers wearing asbestos suits had to clean and maintain the inside of the fireboxes while they were still hot. However, the EMD F7 was soon introduced in passenger service, prompting the NYC to make comparisons with the Niagaras and the diesel locomotives. Although the steam locomotives were only slightly behind the diesel locomotives in terms of overall costs, dieselization could no longer be stopped.

As early as 1953, the most important passenger trains had been switched to diesel. The Niagaras were now mainly used with fast freight trains, and they also achieved good performances. However, their size and axle load limited the area in which they could be used. Their service ended in 1955 and 1956. Since Alfred E. Perlman, then president of the NYC, pursued the strategy of unconditional scrapping, not a single Niagara was preserved.