The history of the New York City transit system teaches us that the original trains were all elevated or an EL Train as it is called. Here is an old car on the now defunct Third Avenue El as it passes apartment building windows in Manhattan.
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The advent of the new underground system in the early 1900s was a game changer in the efficiency and effectiveness of the NYC subway. We still have some El lines today but the vast majority are deep below the ground and have no impact on driving or pollution as it did back in the day.