Sentence:  Where the traffic over a railroad crossing consisted of 5 locomotive round-trips weekly and 1,275 motor vehicles daily, the Commission held that public safety required not only standard reflector-type advance and <WSD>crossback</WSD> crossing warning  signals be erected, but that a member of the train crew precede each locomotive, car or train across the highway and warn the traveling public. 
Meaning of the word marked with <WSD> tags:  An "X"-shaped railroad crossing sign.