Airtrain opened in 2003 [1]. It connected to the subway system and, through Jamaica, Penn Station. The novel bit in 2023 was also linking into Grand Central.

[1] https://en.wikipedia.org/wiki/AirTrain_JFK

Huh. Google Maps was not showing me that.

EDIT: Oh, wait, misread, I thought you meant a direct train from Grand Central to the airport.