PENNSYLVANIA RAILROAD STEAM ENGINE, Pennsylvania Railroad class M1a Steam Engine at the 1939 World's Fair,

Pennsylvania Railroad class M1a Steam Engine at the 1939 World's Fair,

Gottscho-Schleisner collection

PENNSYLVANIA RAILROAD M1 STEAM ENGINE Locomotive

1939 worlds Fair exhibit 

 The M1 class of heavy mixed-traffic locomotives of the 4-8-2  "Mountain" arrangement, which uses four pairs of driving wheels with a four-wheel guiding truck in front for stability at speed and a two-wheel trailing truck to support the large firebox  needed for sustained power.
Although built for both passenger and freight work, they spent most of their service lives hauling heavy high-speed freight trains.
Many PRR men counted the M1 class locomotives as the best steam locomotives the railroad ever owned.

Pennsylvania Railroad class M1a Steam Engine wheel detail  at the 1939 World's Fair,

PRR PENNSYLVANIA RAILROAD 
M1 STEAM ENGINE Locomotive