It was a collision between a T-Third train and a train on the N-Judah line.
Sixteen people were injured when the trains crashed into each other at 4th Street and King.
Twelve of those people were taken to the hospital, but no word yet on just how serious the injures are.
Investigators say the T-Third train was traveling west bound on the King Street tracks at around 2:00 p.m. on Saturday afternoon. The N-Judah train had stopped at the red light as the operator of the T-Third was traveling at normal speeds but had trouble stopping in time.
"We've been told the operator prepared to stop, however the train surged forward and that train rear ended the train that stopped at the red light," says Capt. David Lazar with the SFPD.
"It was a strong impact you could see that things were shattered . There was a ton of smoke and there was a huge boom and everyone came flooding out of the buildings," said Greg Murray, Re-Do Salon.
It's a busy night at the ballpark as the the Giants host the A's at AT&T Park.
Muni does not have a timeline on what the track is going to be operational. SF Muni says that the tracks are closed, however there were a few trains that were running with people on board. Even if those trains are not working, they have set up shuttle busses for people still trying to get here for the game.