Built by Baldwin in 1929, and used by the Southwest Forest Lumber Mills, it is now displayed near the Museum of Northern Arizona in Flagstaff. (Geoff Churchman pic)
from SteamLocomotive.com:
The first Mallet was an 0-6-6-0. Around 1905 the Great Northern Railroad was interested in one but wanted a locomotive that was better suited to main line operation. Baldwin built five "0-6-6-0"s with leading and trailing two-wheel trucks. This configuration suited the GN's curving main line of the Cascades.
It wasn't until 1910 that 2-6-6-2s were built with their firebox behind the drivers and supported by the trailing truck which is normally why one would have a trailing truck. Subsequent 2-6-6-2s were built following this design.
The 2-6-6-2 was primarily a low-speed locomotive. This wheel arrangement was also commonly used on the logging railroads in the Western USA. However, the C&O (Chesapeake & Ohio) continued developing the 2-6-6-2. Their versions evolved into very large road locomotives. With their smokebox-mounted air pumps, they looked quite impressive. Fortunately, two of these class H-6 C&O locomotives survive today.
This wheel arrangement was also used by the Southern Pacific in their development of the Cab Forward locomotive. On the SP this wheel arrangement was called a "Mallet Mogul".
