Its the same antenna (and same protocol I bet) used for CBTC. Its probably used for attaching driver IDs to train fleet nums to train IDs (runs) for Bluetooth Beacon system. CBTC would require wiring in the signals/relay cabinets/interlockings on 4th ave, which I can't tell if that was ever done. With the old style of Itrac a dispatcher would log in the train ID and train fleet number into a desktop after getting the info by radio from the T/O. Often the dispatchers were lazy, or wanted to do some "statistics" optimization, and created "ghost trains" on the countdown clocks, and trains would arrive at origin+1 or origin+2 with no countdown clock info. itrac or MTA policy also allows writing the departure time, on paper by a dispatcher, and hours/days later entering it into itrac, with the original real origin departure time unknown. Post Byford/post Covid, I see trains leaving origin 1-2 mins early, never late. Pre Byford, ghost trains on count down clocks (counts down, nothing comes), or untracked trains were common. Don't see that in 2023 anymore.