The improved South Branch will knock an average of 20 minutes off of people's daily commutes.
The makeover, which began last May, shut down a 10-mile stretch of stops from Cermak to 95th. CTA tried to fill the gaps with shuttle buses and by running Red Line trains on a portion of the Green Line.
"It's been really kind of hard. The buses are always jam-packed and late. So I'm kind of glad the Red Line will be up and running," said Shawanakee Jackson.
"Nothing beats the train. The train gets you there, comes on time. I don't like the buses," said Stephen Harris, CTA rider.
Chicago Mayor Rahm Emanuel and others celebrated the on-time completion of the $425 million renovation project on Thursday.