CHICAGO (CBS) — Four teenagers are in custody, accused in a string of armed robberies at a South Side Metra station.

As WBBM Newsradio’s Bernie Tafoya reports, four older teens and a 15-year-old boy were charged Sunday night with a series of holdups on or near the Metra Rock Island Line’s Gresham station at 87th and Halsted streets.

The break in the case came after police caught two of the teens allegedly showing off a gun at a school playground.

LISTEN: WBBM Newsradio’s Bernie Tafoya reports

Donquell Tillman, 19, and Eric Vaughn, 15, were sitting on a bench at Walter Q. Gresham Elementary School, 8524 S. Green St., Saturday afternoon when officers spotted Tillman showing his .380-caliber gun to Vaughn, Assistant Cook County State’s Attorney Sheri Bennet said.

Confronted after he had tucked the weapon into his waistband, Tillman admitted they were about to “do a stain” — street slang for a robbery, according to a police report.

Passengers near Gresham Metra station, just a few yards from the school, had been targeted by the pair on five occasions this month, it is alleged.

Gresham District officers and Metra Police officers arrested the teens at the school and recovered the weapon after they set up surveillance near the station, according to a police News Affairs release.

Both teens were positively identified in lineups, the release said. The investigation led detectives to two more suspects — James McGill, 18, of the 500 block of West 87th Street and Tommie Gilmore, also 18, of the 8500 block of South Sangamon Street.

On Oct. 1, Tillman and Vaughn allegedly teamed up with four other robbers to beat a man and steal his phone, cash and credit card at the Gresham Metra stop.

Two days later Vaughn and two other robbers stuck a gun in a woman’s abdomen and demanded her purse and phone, Bennet said.

On Oct. 11, Tillman and five others robbed a man, she said. And on Oct. 18, Tillman and three others stole a man’s phone, iPod and cash, then took $3,000 cash from a second man.

All four teens are charged with robbery and armed robbery with a firearm, according to the release. Tillman, of the 8200 block of South Woodlawn Avenue, was also charged with aggravated battery and unlawful possession of a firearm.

Cook County Judge Adam Bourgeois set bond for Tillman and Vaughn at $300,000 Sunday, telling them, “Gentlemen, welcome to the show.”

McGill and Gilmore are expected to appear in bond court later Monday.

The Sun-Times Media Wire contributed to this report.