The success of
the M1 Mountain-type locomotives prompted the Pennsylvania Railroad to
order 100 more locomotives of the 4-8-2 wheel arrangement in 1930, though
several upgrades were incorporated in the new machines. The new
locomotives became the M1a class and had a longer wheelbase than the M1's
due to the addition of the Worthington feedwater heater. They also
received the new larger class 210F75 tenders, which became known as the
coast-to-coast tenders. The first 50 M1a locomotives (#6700 - 6749)
were built by Baldwin; Juniata Shops (later known as Altoona Works) built
the next 25 M1a's (#6750-6774), and the remaining 25 were built by Lima
and numbered 6775-6799.
1946, 41 of the M1a's received further improvements by increasing the
boiler pressure and the addition of circulators to the firebox. The
improved M1a's became the M1b class. The best way to tell them apart
is to look for the circulators on the firebox, above the running boards -
those round objects in a diagonal line that look like wash-out plugs.
4-8-2's were considered to be the finest of the Pennsy's steam
locomotives. They were designed as dual service locomotives and were put
to the test hauling passenger trains, general freight, and in the end even
the heavy ore trains, which there were never intended to pull. They
did it all in style!