Those are call-on switches. a lever on the switch machine turns on an extra white light at the bottom. train pulls up to the signal, pushed the call-on switch, and the stop arm drops.
It's sort of like keying by, but with prior authoriztation.
(when I worked at the transit museum, on of the other fellows there was an ex-tower man, he showed me how to the Court st. tower worked, and the north track has one of these devices)