Sentence:  Welcome to the subway system of New York, the Metropolitan Transportation Authority, or commonly called the MTA by everyone but the residents who will simply tell you they get on a train or ride the subway because in Manhattan all rail transport is underground unlike the outlying boroughs of Brooklyn, Queens, and the Bronx where the trains climb out of this <WSD>subterrain</WSD> and are raised above ground on rail tracks supported by steel girders, hence the El, common speak for the elevated. 
Meaning of the word marked with <WSD> tags:  An underground region.