capable of traveling and with a tractive force of . In all, 89 of these locomotives were built. The first had a box cab design and were placed in service in 1932. The following year, the last 28 under construction were redesigned to have a streamlined type of cab. Some engines underwent re-gearing for freight service. In 1933, two entirely new locomotives were being planned: the R1 and the GG-1 class. The R-1 had a rigid frame for its four driving axles, while the GG-1 had two frames which were articulated. Both of these prototypes, along with an O-1, a P5A and a K4s steam locomotive underwent exhaustive testing. Testing was conducted over a special section of test track near Claymont, Delaware, and lasted for nearly two years. As a result of these experiments, the GG1 type was chosen and the construction of 57 locomotives was authorized. The first GG1 was finished in April and by August 1935 all 57 were completed. These first GG1 engines were designated for passenger service, while most of the P5A type were made available for freight service. Some of the later-built GG1s were assigned to freight service as well. The total number of GG1s built was 139. They are rated at at speeds of . On August 26, 1999, the U.S. Postal Service issued commemorative 33-cent All Aboard! 20th Century American Trains stamps. These commemorative stamps featured five celebrated American passenger trains from the 1930s and 1940s. One of the five stamps features an image of a GG-1 locomotive pulling the "Congressional Limited Express." The official Pennsylvania State Electric Locomotive is the GG-1 #4859. It received this designation on December 18, 1987, and is currently on display in Harrisburg, Pennsylvania. Diesel In June 1937, the Pennsy acquired its first diesel locomotive: a 600-hp diesel-electric switch engine from Electro-Motive Corporation (EMC), a predecessor of General Motors Electro-Motive Division (EMD). EMC called it an SW model; the railroad dubbed it class ES6. The Pennsy bought its second diesel, another switcher, in October 1941: an EMD NW2 (Pennsy class ES10). Wartime restrictions soon restricted locomotive builders' production of diesels intended for freight service. Still, the Pennsy managed to add 15 Baldwin switchers and one EMD switcher. After the war, the Pennsy began to buy diesel locomotives in earnest. From 1945 through 1947, the railroad bought roughly 100 switcher, freight, and passenger diesels from various builders, then another