The Pennsylvania Railroad's K4 4-6-2 "Pacific" (425 built 1914–1928, PRR Altoona, Baldwin) was its premier passenger-hauling steam locomotive from 1914 through the end of steam on the PRR in 1957. Attempts were made to replace the K4s, including the K5 and the T1 duplex locomotive, but neither was very successful. The K4s hauled the vast majority of express passenger trains until they were replaced by diesel locomotives. The K4s were not powerful enough for the heavier trains they often pulled from the mid-1930s onward, so they were often double- or even triple-headed, sometimes with atlantic class locomotives. This was effective, but expensive, and several crews were needed. The PRR did have the extra locomotives, many having been displaced by electrification east of Harrisburg.