That’s the railroad viaduct on Park Avenue, basically a wall cutting the neighborhood in half. This was constructed by the New York Central Railroad in the late 1800s and extended between 1900 and 1910 when the line was electrified and things generally reconfigured for the new Grand Central Terminal. If you like big walls built of big stone blocks it’s interesting to look at, and carries the dirt of some 130 years of coal, gasoline, and diesel smoke.
It’s not a wall, of course. This is an embankment to carry four tracks, with walls on the sides. You’re only seeing the west half (southbound traffic) of Park Avenue here, the other half is on the other side of the embankment.
The rationale for this portion of the tracks being elevated is in several parts. The land here is somewhat lower than it is to the south, where the tracks are in a tunnel. The difference between Harlem Flats (to revive an old name that is pretty much out of use) and Carnegie Hill (to use a name that was just coming in to use around 1910) is only about thirty feet, but it’s noticeable as you walk north past 96th Street. But there are other hills and valleys. I suspect the driving force was that not very far to the north the line crosses the Harlem River and the railroad engineers did not want to build a tunnel under that tidal strait. If the Park Avenue tunnel continued north of the edge of the hill between 96th and 98th Streets, ramped downward to stay as a tunnel and still underground at the 125th Street station, it would have a steep climb to get over a bridge at 134th Street. Finally, the presence of the coal-burning railroad prior to 1910 had blighted the area immediately next to the tracks, so there was no one there with political pull (i.e., someone not living in a tenement) to insist that the tracks be buried.
No streets are blocked by the tracks. They all continue through as little tunnels:
You really have to pay attention to the lights, both as a pedestrian and as a driver, because you cannot see traffic on the far side of the avenue until it’s right on top of you.
I doubt this structure is going away. It’s a good exercise – for idle time, or maybe to give to students – to think of what you would do if the tracks were suddenly to be buried. You could tear the whole thing down and make this stretch of the avenue look like the part to the south, with wide planted medians. You could turn it into a northern version of the High Line. You could do something in between.