The first Niagara was Class S-1a No. 6000 in 1945; the S-1b (6001-6025) were delivered in 1945-46. The NYC's last steam locomotive was Class S-2 No. 5500; it had Franklin poppet valves. The Niagaras did not have steam domes, as did most steam locomotives, which resulted in a smooth contour along the top of the boiler. A perforated pipe collected steam instead. This was necessary because of the lower loading gauge of the New York Central (15 ft 2 in versus for other American railroads).