These are the tracks of the Ninth Avenue Elevated making an S curve beside Morningside Park—which is what this 1908 postcards says.
To my eyes, it’s difficult to recognize the park of 2017, which is one of the city’s least appreciated but most beautiful. (The bear and fawn statue, the rock formations, the turtles….sigh.)
Here’s a photo very similar to the image in the postcard. RIP Ninth Avenue El, which ceased operation in 1940.