By contrast, the T1s were very successful but suffered from greater maintenance costs, wheel slip due to poor springing, and inexperienced crews.

[2][3] Some inspiration came from the large experimental K29 class Pacific built in 1911 by American Locomotive Company (ALCO).

1737 was the first K4 class locomotive built in May 1914 at PRR's Juniata Shops in Altoona, Pennsylvania with the construction number 2825.

1737 was conservative and included a screw reverse (power reverse would soon be added); a small 70-P-70 tender holding only 7,000 US gallons (26,000 L) of water and 12½ tons of coal, set up for hand-firing; a wooden cowcatcher pilot; a square-cased, old-fashioned headlight and piston tailrods (soon to go).

London and North Eastern Railway Chief Mechanical Engineer Nigel Gresley incorporated much of the boiler design (including the tapered shape) into his famous Class A1 Pacific.

Partly, this was due to extensive testing, but wartime necessitated priority in construction to the L1s Mikado type for freight.

5400-5474 were built by Baldwin The PRR experimented extensively with its K4s fleet, trying out streamlining, poppet valves, smoke deflectors, driving wheel types and others.

This was a very concealing, enveloping streamlined casing that hid most of the functionality of the steam locomotive, leading to its nickname of "The Torpedo" by train crews.

Clay models of Loewy's design streamlined K4s and conventional K4s were tested in a wind tunnel for smoke-lifting ability by Alexander Klemin of the Daniel Guggenheim School of Aeronautics of New York University (CMP).

The South Wind began operations in December 1940, providing streamliner service between Chicago, Illinois, and Miami, Florida.

The Museum cited changes in FRA safety standards and new limits to mainline railway access that would make operation impractical.

[20] On June 25, 2021, the Railroaders Memorial Museum in Altoona announced that it would launch a complete $2.6 million restoration of #1361.

The Museum Chairman is Charles "Wick" Moorman, retired Chairman and CEO of Norfolk Southern Railway, and its Board includes Henry Posner III, President of Railroad Development Corporation of Pittsburgh, head of the Posner Foundation and well-known venture capitalist.

[21] As of December 2022, a new Belpaire firebox was nearing completion, although with thicker steel and other modifications of the 1914 design in order to comply with current federal safety requirements.