CTA track worker Alex Gonzalez was getting a kidney from his co-worker, Lee Stark.
Stark learned of that Gonzalez was diagnosed with stage-four kidney failure and stepped in to help.
ABC7 learned that Wednesday's surgery was a success.
Both men are doing well and are recovering.
Coworkers of Gonzalez and Stark are also stepping up, raising money to help both of them while they're away from work recovering.
You can find the GoFundMe for Gonzales and Stark here.
READ MORE | CTA track worker donating life-saving kidney to colleague: 'It's just amazing'