Sentence:  They had lunch at a sidewalk cafe overlooking the intersection of two broad , busy , unpicturesque streets , and coming home they got lost in the <WSD>Metro</WSD> ; it took them over an hour to get back to the station where they should have changed , in order to take the line that went to the Place Redoute . 
Meaning of the word marked with <WSD> tags:  an electric railway operating below the surface of the ground (usually in a city)