In 1900, Baldwin built six Atlantics for the Philadelphia & Reading which were numbered 322 to 327 and designated class P-3a. Built for firing anthracite, they were designed as Camelbacks with a wide Wootten firebox and cab on top of the boiler in front of the firebox. The grate measured 80.7 square feet and the firebox was made even bigger with a combustion chamber that was 48 inches long. These locomotives were Vauclain compounds with cylinder diameters of 15 and 25 inches.

In 1903 they were simpled with cylinders of 21 inches and the boiler pressure was raised from 200 to 220 psi, creating class P-3c. When it was found out that the large number of small tubes did not deliver very good results, they were rebuilt again in 1913 with fewer tubes of increased diameter. They were retired in 1933.

Another locomotive that was originally designated class P-3b was No. 329. It was another Vauclain compound built in 1902 as a comparison to the class P-4a No. 328 with two simple cylinders. Also in this case the two-cylinder variant was better and so 329 received the same cylinders as 328. Now it was also designated class P-4a. Both became class P-4d in 1914 and 1915 when they received new boilers. Also these were retired in 1933.