That’s a picture of the 23rd Street station of the original IRT subway under construction, probably in 1903. That station is now part of the Lexington Avenue line and you can find that stair, although the passageway that makes up the left side of the photo is partially hidden and partially altered. The platform for the local downtown train is the lower area on the right, with the tracks off to the right of the view.
The first thing that comes to mind looking at that is that we have four visible materials playing structural or semi-structural roles. We have steel in the exposed beams on the right and the encased beams and columns on the left; we have concrete in the stairs, we have masonry (brick and tile) as fireproofing for the steel, and we have glass in the lighted sidewalk above. This is a subway station: the skylight is the sidewalk.
The stair is of interest because a brand-new and little known company – Turner Construction – got the contract to construct all of the reinforced concrete stairs for the IRT stations built 1903-1904. Reinforced concrete work was, in New York at that time, still a specialty trade practiced by a few wizards who knew its mysterious secrets. That would change quickly enough, but by the time concrete had become the local standard for large industrial buildings, at the end of the decade, Turner had moved on from stairs to whole buildings and the company hasn’t looked back since.
I sometimes tell scary (but true) stories about the subway as it was when I became a regular rider in the mid-1970s. It was dark down there, with many station platforms lit by a single incandescent bulb every ten feet. Modern lighting was later installed and it’s a lot brighter now, but this photo is a good reminder that the original design had daylighting in many of the stations. Gradually, at some time before the mid-70s, those lighted sidewalks were replaced by plain concrete slabs, probably as an effort to simplify maintenance and to try to stop leaks. Thanks to Wikipedia’s article on this station, here’s a colorized and only slightly fanciful postcard showing the same view after the subway was in operation:
Pretty swanky. Of course, the daylighting would do no good at night, and those pristine whites and yellows did not long survive dust and a lack of cleaning during maintenance. The passageway on the left can be seen in this picture as doing double duty as a shopping arcade. A number of such underground strip malls were built with the subway or were installed later, but most are gone.