Those killed, both women, were apparently walking near the Cermak-Chinatown Red Line station on the city's South Side, Fire Department spokeswoman Eve Rodriguez said. They were dead at the scene, she said.
Twenty-one people were transported to area hospitals, said fire department spokesman Larry Langford. Eleven were in critical condition, including four children, eight adults were in stable condition and two adults were in good condition.
"This is rush hour so it's bad," Langford said.
Workers were searching through wreckage Friday evening for more injured, Langford said.
Rodriguez did not know the driver's condition, or whether the driver was injured.
Witnesses said the truck didn't appear to slow down before it plowed into the station.
The truck hit pedestrians, then "climbed the stairs" of the station's north stairwell around 5:20 p.m., Langford said.
Hazardous materials crews were on the scene because the semitrailer leaked fuel, Langford said.
Most of those injured were either in the bus shelter underneath the elevated train station or in the station's stairwell, Rodriguez said.
Power was shut off to the area, CTA spokeswoman Catherine Hosinski said.