Via the Transitmuseum.org

New York City’s earliest form of rapid transit was the elevated railway, or el.  The first elevated line with passenger service was the cable-powered West Side and Yonkers Patent Railway, which opened in 1868 and ran for just a few years.  When the New York Elevated Railway introduced small steam locomotives to replace cables in 1871, the age of the els had arrived.  Designed to run on tracks nearly three stories above city avenues, the elevated trains drastically changed the ways in which New Yorkers viewed their city and lived their lives.  By 1880 most Manhattan residents were within a ten-minute walk from an el.  By 1903 the elevated systems in Manhattan and Brooklyn had shifted from steam to electric power, offering a smoother, cleaner ride. The els ushered in aspects of urban life that we now take for granted – from being able to live, work, and shop in different parts of the city, to constantly interacting with people from different neighborhoods and backgrounds.  Although the els were dirty and noisy and blocked sunlight from the streets below, they allowed people to travel quickly and cheaply throughout the city for nearly a hundred years, helping transform New York into a bustling metropolis.

Sixth Avenue El at 42nd. Note the elaborate el platform arching over the intersection. The Sixth Avenue El is  mentioned many times in the novel.

Elevated Train Map ca 1881 Library of Congress