(CNN) -- A fire in Chicago's subway system Sunday left at least 12 people with smoke-related injuries -- five of them in serious condition, according to a fire official.
Railroad ties in a tunnel near the city's Clark Street station caught fire after apparently being ignited by sparks from passing trains, said Larry Langford, a spokesman for the Chicago Fire Department.
Ten ambulances responded to the fire, which poured "heavy smoke," Langford said.
A photo snapped by a passer-by shows thick, black smoke billowing above ground.
Service to the subway's Red Line was disrupted and trains were being rerouted to the system's elevated tracks, the Chicago Transit Authority said.
In addition to the five people listed in serious condition at local hospitals, four others were in stable condition and three in good condition, Langford said.
According to Langford, such fires are not uncommon in Chicago during the summer months when dry conditions and sparks combine, though he noted that railroad-tie fires typically occur on lines above ground.
CNN's Greg Morrison contributed to this report.