Sentence:  American Locomotive Company, which introduced the 4-6-6-4 (on Union Pacific) in 1936, constructed more <WSD>Challengers</WSD> than any other builder and, in Northern Pacific's Z-8 class, built the heaviest. NP owned 48 4-6-6-4's in three classes. 
Meaning of the word marked with <WSD> tags:  a steam locomotive of the 4-6-6-4 wheel arrangement. (4-6-6-4)