A quick-thinking MTA worker in Harlem jumped into action after noticing a person on a No. 4 train was carrying a firearm this morning, police say.

According to authorities, an MTA employee noticed an individual had a firearm on their waistband around 4 a.m. at the 126th Street Station platform.

The worker alerted the police, and the suspect has been arrested.

The MTA took to Twitter to report delays on the northbound No. 4 trains earlier Tuesday as police investigated the matter.

No other information has been released.