PRR S1

The PRR S1 class steam locomotive (nicknamed "The Big Engine") was a single experimental locomotive, the longest and heaviest rigid frame reciprocating steam locomotive ever built. The streamlined Art Deco styled shell of the locomotive was designed by Raymond Loewy.

The S1 was the only locomotive ever built with a 6-4-4-6 wheel arrangement. It was a duplex locomotive, meaning that it had two pairs of cylinders, each driving two pairs of driving wheels. Unlike similar-looking articulated locomotive designs, the driven wheelbase of the S1 was rigid. The S1 was completed January 31, 1939 and was numbered 6100.

At 140 ft 2 1⁄₂ in (42.74 m) over engine and tender the S1 could not negotiate curves on much of the PRR system. This problem, along with wheel slippage, limited the S1's usefulness. No further S1 models were built as focus shifted to the T1 class. The last run for the S1 was in December 1945 and the engine was scrapped in 1949.