Of Mallets and “Mallets”

[rcdrye]

B&O had only two simple 2-6-6-2's.  The 55 ex-BR&P KK-4 class (7500-7554) were compounds and thus Mallets.  The slide valves on the forward cylinders were a KK-4 characteristic.

[BaltACD]

rcdrye

B&O had only two simple 2-6-6-2's.  The 55 ex-BR&P KK-4 class (7500-7554) were compounds and thus Mallets.  The slide valves on the forward cylinders were a KK-4 characteristic.

Per Lawrence Sagle's "Picture History of B&O Motive Power", the simple KK-1 & KK-2 were constructed to test the differences between the Emerson water-tube boiler and the conventional radial-stayed firebox.  Both were built by Baldwin in 1932, (the same year the B&O acquired the BR&P).  As of January 1, 1952 the book reports that only the radial-stayed example was still operating.  Of the BR&P examples only 4 of them were still in operation on January 1, 1952.

[Overmod]

The locomotives that became the KK-1 and 2 are famous for being the first true high-speed simple articulateds in the world.  The wheel arrangement was not really more satisfactory than a double Prairie, though; B&O heroically tried substituting a 4-4-0 forward engine, but the cure appeared to be worse than the disease.

The evolved form, though, relatively quickly appeared as 'one-and-a-half AMC Berkshires' once people realized how to make two-wheel lead trucks fast; the answer with proper 4-wheel trailer was the 2-6-6-4 (after a little time with drag-freight design as on P&WV) and of course for high horsepower and acceptable water rate the 'double sixes' were as many drivers as needed for a single locomotive, even to the size six-wheel trailing trucks were used...

B&O bought some 2-6-6-4s late -- used, but they were good stock -- some sources say they were among the best steam power to run on that railroad, and that is in august company.