Sentence:  These 4-8-2's, of which B&M eventually purchased 18 in various <WSD>subclasses</WSD>, could almost haul the tonnage of a T and more than made up for the slight deficiency by raising the maximum speed limit from 45 to 70 miles per hour, thanks to 73-inch drivers. 
Meaning of the word marked with <WSD> tags:  A secondary class within a main class.