An 18-year-old man accused of fatally pushing a Marine veteran in front of a train in Chicago is now facing first-degree murder charges.

Police arrested Ryan Munn Friday night and charged him with first-degree murder for allegedly pushing Mamadou Balde, 29, in front of a CTA Red Line train on Tuesday, WGN reported.

Munn was identified by witnesses as he “punched and pushed” Balde into the passing train, according to the police report.

The arrest is Munn’s first, and he is scheduled to appear before a judge for his first bond hearing on Sunday, the Chicago Tribune reported.

“It would be very, very good if this happened quickly because it’s just too much,’’ Mamadou Balde’s father, Al Balde, said early on Saturday afternoon on learning of the arrest. “Someone threw someone alive into a train. This is crazy.”

Balde was waiting for a train Tuesday evening at the Jackson Red Line Stop when three people began arguing with him, police said.

Surveillance video reportedly showed Balde walking away from the squabble when one of the three men ran up to him and pushed him off the platform into a moving train, killing him instantly.

Balde, who served for two tours in Afghanistan, was found dead by the time the police arrived at the CTA Red Line station.

The Cook County Medical Examiner said Balde died from multiple injuries.

The attack was one of eight homicides recorded in Chicago on Tuesday, marking the deadliest day of violence the city has seen in one day for nearly two years.