HO4-6-6-4 CSA-1 Challenger Locomotive, UP #3907 Overview
ALCO constructed the first 4-6-6-4 locomotives in the world, numbered 3900 to 3914, for the Union Pacific Railroad in 1936. These huge engines were named the Challenger Simple Articulated, or CSA-1, class. The railroad received 25 near copies, classified as CSA-2, the following year.
Unlike compound Mallet engines that crawled along with drag freights, the Challengers had two sets of high- pressure cylinders and could safely reach passenger train speeds. The mammoth locomotives also had a heavy-duty starting tractive effort of 97,400 pounds.
Originally constructed to haul fast freight up the famous Wasatch grade, the CSA-1 and -2 Challengers would hold a variety of assignments over their long careers- including passenger service. Depending upon their operating territory, they burned either coal or oil, and some locos were converted back- and-forth several times.