There are eight new stops along six miles of track. The nearly $900 million project took about five years to build and was many years in the planning.
"This is the most transit-dependent community in L.A. County. It will give them access to jobs, schools, medical care and other opportunities. There's going to be development here, there's going to be housing and business opportunities," said MTA spokesperson Marc Littman.
"We're so happy that we have new train here. I live three blocks from here, and it's very useful transportation," said Diana Cerillo, a Gold Line rider.
Above ground, the trains will travel up to 35 mph, but the 1.7 miles of underground track allows the trains to travel up to 55 mph.
Metro says that it will have safety ambassadors on duty at crossings and they will install security cameras. Drivers caught going around the crossing arms face a $450 fine.
The MTA estimates 13,000 people will ride the Eastside Extension on weekdays.
"I think it's terrific. It's less traffic on the freeways, and it's all good," said Bob Westman, a Gold Line rider.
"Metro has really cleaned things up. We were in New York this summer, and it's quite a different story on the subways back there," Westman added.
The trains will run every eight minutes during the morning commute, but the free ride is only good for Sunday.