Class D16 on the Pennsylvania Railroad was their final development of the 4-4-0 "American" type of steam locomotive. A total of 429 of these locomotives were built at the PRR's Juniata Shops, spread across five subclasses; some had diameter driving wheels for service in level territory, while others had drivers for mountainous terrain. In the pre-1895 scheme, these locomotives were second class L. Construction continued until 1910, and the locomotives, aided by a rebuild program from 1914, remained in service in large numbers until the 1930s, a small number surviving into the 1940s. One locomotive, #1223, was preserved and is currently on display. Design These locomotives were originally conceived as an enlargement of the earlier class P (later reclassified D14) and were an extremely large and powerful locomotive for the period. Breaking with the traditional 4-4-0 layout with a low-slung boiler and the firebox between the frames, the class L design had a large Belpaire firebox above the frames and a large high-mounted boiler. The high center of gravity proved to offer an exceptional high-speed ride. The design was the product of three men; general superintendent of motive power Frank D. Casaneve, chief mechanical engineer Axel S. Vogt, and chief of motive power Theodore N. Ely, Casaneve supervising the overall design, Vogt perfecting the mechanical details and Ely paying more attention to the appearance and external detail. Two versions were conceived, reflecting the variety of terrain the PRR traversed; a high-drivered version for flat terrain with wheels and a low-drivered version for hilly terrain with wheels. The versions had tractive effort ratings of and , respectively. In the reclassification of 1895, the 68-inch drivered locomotives became class D16 and the 80-inch became D16a. Production 73 of the high-drivered D16a subclass were built between 1895–1898, and 6 of class D16 in 1896. A slightly revised low-drivered subclass D16b was constructed to the tune of 262 examples between 1900–1908, as well as 12 high-drivered D16c in 1900 and 45 high-drivered D16d between 1900–1910. As the American (4-4-0) type was displaced from top-flight service to secondary duties, tractive effort became more important than top speed. Large numbers of the high-drivered locomotives were converted to low-drivered; 76 D16a and D16c were converted to subclass D16, while 9 of class D16d were converted to D16b specification; this left only a small number of locomotives with 80-inch drivers. Rebuilds In 1914, the PRR experimentally rebuilt D16b