The above video shows the subway line from Union Square to Grand Central, in 1905. This would be the route taken by the 4, 5, and 6 trains, today; the Lexington Avenue IRT. According to the Library of Congress:
The camera platform was on the front of a New York subway train following another train on the same track. Lighting is provided by a specially constructed work car on a parallel track. At the time of filming, the subway was only seven months old, having opened on October 27, 1904. The ride begins at 14th Street (Union Square) following the route of today’s east side IRT, and ends at the old Grand Central Station, built by Cornelius Vanderbilt in 1869. The Grand Central Station in use today was not completed until 1913.
The clothing at the end is incredible. After the route covered by the video, the train would have turned west, and followed the shuttle tracks to the West Side, where it would have continued north along the tracks now used by the 1, 2, and 3 into Harlem and Washington Heights, and eventually the Bronx. This was before there were tunnels under the rivers to Brooklyn, Queens, or Jersey City. As the subway grew northward, it would include architecturally unique stations like the one at 168th Street, whose design echoed the tepidarium of the Pompeiian baths:
(It’s interesting how much more front-and-center the references to the Classical world once were in American city planning.) In spite of being the only subway, the first line existed in the context of an established and extensive elevated system, which had provided above-ground urban rail to New Yorkers since the mid-Victorian period; and also electric streetcars. The NYC video is kind of like a subterranean version of the below movie, which was filmed from a San Francisco streetcar, on Market Street, traveling towards the Embarcadero, just days before the infamous earthquake in 1906:
This is the route now followed by the underground BART. Great stuff.