Sentence:  But whenever one of the motor-trucks lumbering by bore a big U.S. on its rear panel Troy pushed his light ambulance ahead and skimmed past, just for the joy of seeing the fresh young heads rising pyramid-wise about the sides of the <WSD>lorry</WSD>, hearing the snatches of familiar songs—"Hail, hail, the gang's all here!" and "We won't come back till it's over over here!"—and shouting back in reply to a stentorian "Hi, kid, beat it!", "Bet your life I will, old man!" 
Meaning of the word marked with <WSD> tags:  A motor vehicle for transporting goods, and in some cases people; a truck.