Sentence:  When an engineer wished to stop a swiftly moving train he had first to whistle to the <WSD>guard</WSD> requesting him to apply the hand-brake of the van, and then apply the hand-brake of the engine. <WSD>Guards</WSD> did not always hear. 
Meaning of the word marked with <WSD> tags:  An employee, normally travelling in the last vehicle of a train, responsible for the safety of the train.