The K4s was the Pennsylvania Railroad's premier passenger locomotive, during the later days of steam. 425 of these heavy 4-6-2's ("Pacific" type) were built in the years between 1914 and 1928.

The K4s evolved out of several other types of PRR locomotives, most importantly the experimental K29 Pacific, and the PRR's fleet of giant 4-4-2 "Atlantic" types, the E6s.

It is important to note that the PRR's motive power department tended to be very conservative in nature, and rather than utilizing newer wheel arrangements, tended to build bigger and better versions of old ones. The E6s Atlantic was able to outperform a light Pacific, so long as the train length did not exceed six cars.

The K29 was ordered from the American Locomotive Company (Alco) in 1911. While the K29 was not particularly successful, due to a poorly designed boiler, the PRR's locomotive designers were able to adapt the design, minus the flaws, for the K4s. The K29 weighed 317,000 pounds, and generated a tractive effort of 43,375 pounds. (The Pennsylvania's early Pacific designs, the light K2 and K3 Pacifics, only generated 32,620 pounds of TE.) The K29 spent her entire service life hauling trains over the mountain grades between Altoona and Pittsburgh, and was retired in 1929.

The first K4s, #1737, was built in 1914 by the railroad's own Altoona shops. The design was very similar to the K29, but the locomotive was lighter by 12,000 pounds, and actually generated a slightly greater tractive effort. #1737 spent three years being tested in road service between Harrisburg and Pittsburgh before the go-ahead was given for fleet production.

324 more locomotives were built at Altoona between 1917 and 1924, and an order of 100 more was placed in 1927. Of the final 100, 75 were built by the Baldwin Locomotive Works. The last 100 were built at a time when the K4s design was no longer state-of-the-art and seemed to be another example of PRR conservative thinking.

In the 1920s and 1930s, the K4s was the last word in passenger power on the PRR. It hauled all of the PRR's famous limited trains, including the Broadway Limited. Several locomotives were streamlined for service on the Broadway, the South Wind, and the Spirit of St. Louis. Other locomotives were slightly rebuilt with experimental features and reclassified as K4sa, K4sb, etc. The K4s was finally superseded in the late 1940s by diesel locomotives and by the T1 class Duplex locomotive. The K4s remained in mainline passenger service for several years, as the diesel locomotives were unreliable at first, and the T1 was something of a failure, but by 1947, the PRR had begun to sell K4s locomotives for scrap.

Incidentally, many K4s locomotives saw service on the Long Island Rail Road, which was under PRR control between 1910 and 1950.

The K4s spent its last days in commuter service on the New York & Long Branch, operated today by New Jersey Transit as their North Jersey Coast Line. Approximately 30 were still in service in 1957, their final year of operation.

Two K4s locomotives survive to this day. #3750 is at the Pennsylvania Railroad Museum in Strasburg, Pennsylvania. It is not believed to be operational. #1361 is currently in the Steamtown shop in Scranton, Pennsylvania, being rebuilt for a possible return to excursion service. #1361 was displayed for many years near the trackside at Horseshoe Curve, near Altoona, before it was restored in 1987. The 1361 made a few excursion trips, one of which I rode in York, Pennsylvania, in the late 1980s, but suffered from the problem that many operational steam locomotives in the US have. Firstly, the insurance industry requires a ridiculous amount of coverage for any mainline trips, and secondly, very few large railroads are receptive to steam locomotives running on their tracks. So the 1361 ran a few short and forgettable trips on short and forgettable railroads in western Pennsylvania, before once again sitting idle. Once the rebuilding is complete, the 1361 will probably see some service on the Steamtown trackage between Scranton and Moscow, PA, but faces an uncertain future beyond that.

Source: Classic Power 6 (the many faces of the Pennsy K4s) by Bert Pennypacker and Alvin Staufer. Published by N.J. International of Hicksville, NY.