143rd Street Building
This roofless relic of a building sits between Riverside Drive and the train tracks at 143rd Street. I'm not sure what it was and cannot find any references to it on the web. It has no doors indicating that it was a comfort station and it may have been a storage or service building of some kind. From the architecture, it obviously pre-dates the 1934 improvement of the park and likely is from the late 19th or early 20th century.
The building sits just to the south of what is now the 145th Street viaduct leading into Riverbank State Park. Under the viaduct is an closed passageway that probably once carried 145th Street under Riverside Drive and into the park.