The injured individuals were doing maintenance work in a ComEd substation that is connected with the CTA at 10 East Lake Street, just east of North State Street. It happened in a two-story structure behind the Wit hotel.
Around 11 a.m., an electrical panel "arced," causing the flash fire.
Two CTA workers close to the transformer suffered "significant burns," and were transported in critical condition to Northwestern Memorial Hospital, the CFD said.
A third worker was less seriously injured. That worker was also taken to Northwestern, where his or her condition stabilized. He was treated and released Monday afternoon.
ComEd officials said 500 customers lost power due to the fire. CTA service was not impacted.
The substation provides power to the nearby elevated trains. CTA used an alternative power source to keep the trains running.
Emergency crews were at the scene until around 12:50 p.m. State was blocked to traffic between East Upper Wacker Drive and East Randolph Street. Lake, which is one way in this area, was blocked to traffic at North Dearborn Street.
RIGHT NOW: Large CFD response @ State and Lake, focused on alley north of intersection. Will update w/latest. pic.twitter.com/51uhEDJnPg— Jesse Kirsch (@JesseKirschABC7) May 1, 2018