Underhill titled the picture above “Rapid transit construction work at Union Square, New York City, June 8, 1901” which is about as thorough as you need, but there are a lot of interesting historical niches associated with it. Starting with the surroundings, there are three visible buildings facing Union Square that are included in The Structure of Skyscrapers: the Decker and Hartford Buildings on the left (helpfully painted with their names) and the Jackson Building (the tall narrow one on 17th Street, in the center group). 14th Street is where the numbered street grid, which is a bit shaky to the south, finally gets straightened out, and you can see this here as Fourth Avenue, the diagonal on the lower right, curves to run north parallel to the other avenues. The equestrian Statue is George Washington (and his horse), and was moved about twenty years later from Fourth Avenue to the middle of the park.
The rapid transit was the IRT, the first subway in New York, and it was built cut-and-cover wherever possible. The portion of Fourth Avenue that was still open when the picture was taken is supported at the edge of the cut by wood sheeting supported by heavy-timber walers and rakers, which was standard for the era. Most of the traffic on the thin remaining strip of Fourth is streetcars, with a few horse-drawn vehicles interspersed. There are a lot of small cranes – it looks like three between 14th Street (where Underhill was standing) to 15th Street (just beyond the three streetcars clustered together) – to distribute steel down to the construction below.
The platforms of the IRT had to be lengthened for bigger trains shortly after it was constructed, because the subway was more popular than predicted. Since the line followed Fourth Avenue around the curve, this meant that the station platforms ended up with curves, which is far from ideal. Trains bend at the joints between cars, so when you have a station on a curve, the gap between the edge of the car (and therefore the car doors) and the platform will vary, being greatest at the middle and least at the ends. The solution in the 1910s was to install moving “gap fillers” to reduce the gap for the center doors of the trains. When the train is moving, the retract until they are flush with the edge of the platform; when the train is in the station, they extend to reduce the gap. It’s a kludge, but one that’s been working for around 110 years so it’s got to be classified as a successful kludge. Here’s one in operation:
