No injuries were reported and about 100 passengers were evacuated from the train.
The Ventura line 906 train went off the tracks at 4:38 p.m. just a few hundred yards before pulling into Union Station, Metrolink spokesman Jeff Lustgarten said.
The immobilized train -- which regularly travels between Burbank and downtown Los Angeles -- is blocking three tracks, hampering inbound and outbound rail traffic for evening riders.
At least 20 commuter trains that normally travel on five tracks must now move on two, and at slower speeds than normal. Delays could last between 30 minutes to an hour.
"We're moving trains as quickly as we can," Lustgarten said.
The cause of the derailment is unclear.