A special team assembled by the sheriff's department arrested 33-year-old Gene Sim in a Buena Park CVS parking lot at 9:05 p.m. Tuesday.
Sim allegedly stabbed Jesse Garay of North Hollywood after an altercation on Friday night on the Metro Red Line as it was arriving at the Vine Street station in Hollywood.
A witness said Garay was swinging a chain that turned out to be plastic. It slapped one of the skateboards that Sim was holding.
"They were both in each other's face. One of them looked like he was intoxicated, you know, so honestly, I didn't even want to get in the middle of it," said witness Robyn Peterson.
In moments, passengers allegedly saw Sim pull out a knife and stabbed Garay.
It was the first-ever murder on the Red Line, which has been open for 17 years.
"Everybody ran upstairs, freaked out, so it was crazy," Peterson said.
Police say Sim has a history of arrests. Within the last two months, he was arrested for narcotics and then for assault. He was in court just last week and was ordered to a drug rehab program.
Sim was a transient after he failed to pay rent at a Koreatown apartment. Neighbor Paul Yi said Sim was a night owl and described him as restless.
"He couldn't stay still, and he's just always thinking that something's going to happen," Yi said.
Investigators said they knew who they were looking for early on because there was surveillance tape and they had booking photos from earlier arrests.
Police said they did not release photos of Sim because they feared he would flee if he knew detectives were on his trail.
Sim was booked for probation violation and is being held without bail. The district attorney is evaluating the evidence to determine whether Sim should be charged in the case.
Mayor Antonio Villaraigosa, who also chairs Metro, praised the sheriff's for the arrest.
"I want subway riders to know they are safe. Our transit system as well as our streets and neighborhoods are the safest they've been in a generation," Villaraigosa said in a statement.