Sentence:  One of the first standard types of automated visual grade-crossing warning was the automated flagman, a signal commonly known as a "<WSD>wigwag</WSD>." According to Santa Fe documents, the <WSD>wigwag</WSD> was adopted as a standard crossing device by the American Railway Association in 1923. A standard <WSD>wigwag</WSD> is actuated by a track circuit and consists of a paddle with a red lamp that gracefully swings back and forth in a horizontal pattern when a train approaches. A <WSD>wigwag</WSD> is usually accompanied by a bell. 
Meaning of the word marked with <WSD> tags:  A grade crossing signal with a swinging motion used to indicate an approaching train.