Fair of 1939-40, K5 5698 was displayed along with several other PRR locomotives at the fair. In 1941, following the fair, 5698 was modified with a smokebox extension and a different feedwater heater within. 5698 was again modified by 1948 with a new (one-piece) smoke box, new cylinders with outside steam pipes, and “modern” front end. Baldwin-built #5699 In contrast, Baldwin-built #5699 (Baldwin 60660 / 1928) used a conventional, fabricated locomotive frame, longer smokebox with the feedwater mixing chamber in front of the stack and the generator mounted on the right side (below the stack), and separate cylinder plus half-saddle castings. The innovation trialled on #5699 was its use of Caprotti valve gear, a poppet valve gear invented in Italy. The greater efficiencies of this gear gave the locomotive a higher tractive effort of . Although efficient, the Caprotti gear proved less reliable than the regular Walschaerts valve gear and required more frequent, more skilled maintenance, and in 1937 it was replaced by Walschaerts gear driving regular piston valves, making the two locomotives (except for frames) functionally identical. Service life The two locomotives were originally assigned to haul Philadelphia to Pittsburgh through trains. After the electrification reached Harrisburg, they were reassigned to the twisting Harrisburg–Baltimore route. Finally, they were reassigned to Pittsburgh–Crestline trains. Both were updated after World War 2 with the standard front-end "beauty treatment" given to most K4s locomotives: a sheet-steel drop coupler pilot, higher-mounted headlight, and turbo-generator mounted on the smokebox front for easier access. They lost their unique bell placement at this time for a location equally unique for the PRR: under the generator-service platform on the front of the smokebox. Conclusion Like many experimental locomotives, the K5 was only a qualified success. Both locomotives performed well enough to be kept in service, but they did not perform so outstandingly as to persuade the PRR to build more. They proved that building a larger Pacific than the K4s was not worthwhile; the reduced factor of adhesion limited the locomotives' ability to put their greater power to full use. A larger locomotive with eight driving wheels was needed. This route was eventually taken, although in striking form with the T1 4-4-4-4 duplex locomotive, but the Great Depression and the huge cost of electrification meant that PRR steam locomotive development effectively ceased after the K5 and M1a. The electrical department became the thrust of PRR locomotive policy, and