Sentence:  Indeed, development proceeded so rapidly thereafter that Mopac, for instance, was rebuilding a series of 10-year-old Woodard 2-8-4's into 4-8-4's by 1940 - not because they couldn't steam but because their 63-inch drivers couldn't roll the <WSD>hotshots</WSD> fast enough. 
Meaning of the word marked with <WSD> tags:  A fast freight train.