The Heavy Pacific was intended as the standard heavy express locomotive in the USRA program. It shared the cross-section of the boiler barrel with the Light Mikado and Light Mountain. It had a driver diameter of 79 inches (2,007 mm) and cylinders with a 27 inch diameter and 28 inch stroke.
Only 20 of the original USRA design were delivered to the Erie Railroad and designated class K-5. 20 P-7 from the Baltimore & Ohio, 64 Ps-4 from the Southern and two P-74 from the West Point Route were built as copies, some of which had a different driver diameter. Today, one locomotive still exists of each of the three classes of copies. 