The Pennsylvania Railroad's class A1 was a class of 0-4-0 type steam locomotives.
The Pennsylvania Railroad's class A1 was a class of 0-4-0 switcher type steam locomotive. When the A1 was introduced into service, 0-4-0s were being used by other railroads. In time the 0-6-0 came and other railroads neglected the 0-4-0. But, on the Pennsylvania Railroad, this wasn't happening. This is because, the PRR had a bunch of tight trackage and street trackage as well. The Pennsylvania Railroad elected to use a small locomotive to operate these tight and confined tracks. By the 1920s the Pennsylvania Railroad class A1 was greatly replaced by the even larger class A5s, the largest 0-4-0s on the Pennsylvania Railroad's roster of 0-4-0s in service at the time. By the 1950s as the diesel switchers became available for easier and more efficient switching duties, the Pennsylvania Railroad started to replace the 0-4-0s and 0-6-0s notably the Pennsylvania Railroad class B6sb switchers with diesel switchers. In 1957, as with all Pennsylvania Railroad steam locomotives still in service, they were completely replaced by the diesel locomotive.
PRR B6 This is about the Pennsylvania Railroad's class of highly successful 0-6-0 type switchers.