B6sb #4033 at Erie, Pennsylvania, 1937. The B6sb subtype was the most numerous and successful of the B6 locomotives. This is a B6sa, #3. Note the radial-stay firebox, round-topped and lacking the square Belpaire "shoulders". This is an original B6. Note the absence of external valve gear; internal Stephenson gear is used. The Pennsylvania Railroad's class B6 was its most successful class of switcher, or as the PRR termed them, "shifter". The PRR preferred the 0-6-0 wheel arrangement for larger switchers, whereas on other roads the 0-8-0 gained preference. The PRR used road locomotives, generally 2-8-0s, when larger power was required.
The original class B6 was built at the Altoona Works during 1902–1913; 79 were constructed. These had the PRR's trademark square-shouldered Belpaire firebox and 56 inch drivers. They were constructed as saturated steam engines, rebuilt with superheaters later as class B6s, and had piston valves and Stephenson valve gear.
The next version built was the B6sa, 55 of which were built at Altoona during 1913–1914. These had radial-stay fireboxes, common elsewhere but rare on the Pennsylvania, and they replaced the Stephenson gear with the more modern Walschaerts valve gear. Steam delivery pipes were outside, like all other PRR modern power. All were built superheated.
Finally, during 1916–1920, 238 of class B6sb were built. These were the final, definitive type, and had a Belpaire firebox, but were otherwise little changed from the B6sa. The final 97 locomotives had piston valves mounted outboard of the cylinders, instead of inbound as previously built, giving the cylinder assemblies an outward cant at the top, rather than inward.
All B6sa and B6sb locomotives were retrofitted with power reverse to make the frequent back-and-forth of switching quicker and easier.
The last PRR locomotive in active service was #5244, leased to Union Transportation of New Egypt, New Jersey until July 1959.
One B6 survives, B6sb #1670. It was saved in the PRR's historic collection at Northumberland, Pennsylvania and was donated to the Railroad Museum of Pennsylvania along with the majority of that collection.
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