Superintendent L.B. Paxson of the Philadelphia & Reading was convinced in the mid-1890s that an express locomotive with one powered axle would still be sufficient to haul express trains at high speed. He was thinking in particular of some trains between Jersey City and Philadelphia that covered the 90 miles with a light load. When asked about the ideal wheel arrangement, the choice fell on 4-2-2, which was popular in Great Britain and was called “Single” or “Bicycle” in the US.
A Wootten firebox with a grate of 75 square feet was installed in order to be able to produce a lot of steam at high speeds with the combustion of residues from the anthracite processing. This meant that it was designed as a Camelback locomotive with the cab in the middle of the boiler. The steam dome sat behind the cab on the firebox. The power was provided by a four-cylinder compound engine of the Vauclain type, in which a jointly cast high and low pressure cylinder could be found on each side.
After the number 385 built in 1895, the almost identical number 378 followed in the following year. The locomotives completed the mentioned line including the intermediate stops at an average of 51.4 mph and ran very smoothly. The 385 is said to have even reached 120 mph for a short time what is disputable. Even though the two locomotives easily hauled trains of around 350 tons, the low adhesive weight was soon no longer sufficient.
Both were rebuilt in 1904 into locomotives with a 4-4-0 wheel arrangement, which differed from each other. One thing they had in common, however, was that both received a simple two-cylinder engine with 19 by 26-inch cylinders. 385 became class D10-a and retained its 84.25 inch driver diameter. It lost its combustion chamber during the rebuild and instead got a larger tube heating surface. 378, now class D10-b, had the drivers reduced to 78.5 inches and still had a combustion chamber, but also had a slightly larger boiler overall.
