A gun has been taken off the streets, thanks to a quick-thinking Long Island cop who overheard two riders aboard a Long Island Rail Road train talking about buying and selling a firearm, law enforcement sources said Tuesday.

Officer Alexander Seda, a former NYPD cop who is now a rookie in the Suffolk County Police Department, was aboard a Brooklyn-bound train on his way to court in connection with an old case when he overheard the men’s conversation Monday afternoon, sources said.

Seda then notified cops from his former precinct — the 79th Precinct in Bedford-Stuyvesant — and also informed the LIRR conductor about what he heard, according to sources.

Cops from the 79th Precinct met the train at New York Avenue and Atlantic Avenue around 2 p.m. and took one of the men — 25-year-old Gregory Chickel — into custody after he was found with a handgun on him, sources said.

Chickel, a Queens resident, was charged with criminal possession of a weapon and criminal possession of marijuana, sources said.