The boiler for the S1 was the largest built by the Pennsylvania Railroad; with 660 sqft of direct heating surface and 500 one-inch diameter tubes and flues, the total heating surface area of S1 was 7,746 sq ft (719.6 m2), it was 99.3% as massive as the boiler for Union Pacific's 4000-class 4-8-8-4 locomotive, the "Big Boy".