Train movements within the subway system used to be controlled by a series of small ‘interlocking towers’. These buildings were located in tunnels and on elevated lines, overlooking junctions where trains could be switched from one track to another.
Most of these old towers have been closed over the years, in favor of centralized dispatching centers.
The old towers still exist though. Some were left as-is, while others, like this one, were stripped of track controls and replaced with electrically panels for tunnel lighting & other utility functions.
Just like in New York Above, space underground is a commodity, and it is rare for any space to be left completely vacant.