Sentence:  I felt superior to the neighborhood friends I was leaving behind , felt older than my years , and was full of compliments for myself as I headed into the <WSD>subway</WSD> that was carrying its packs of passengers out of that dull borough and into the unstable , tantalizing excitement of Manhattan . 
Meaning of the word marked with <WSD> tags:  an electric railway operating below the surface of the ground (usually in a city)