A six-story building that caught fire beside a busy downtown commuter train line was in danger of collapsing Wednesday, forcing the city to shut down the nearby stretch of tracks indefinitely.

The building still had hot spots smoldering Wednesday, said Fire Commissioner Raymond Orozco. It was vacant when the fire broke out Tuesday, and emergency officials planned to bring in a wrecking crew to figure out how to safely tear it down, he said.

"It's going to burn for quite a while," Orozco said. "Part of the building has already collapsed and some of the hot spots are underneath that rubble."

Chicago Transit Authority suspended service indefinitely on the nearby elevated track, which serves two major rail lines, including the line connecting the Loop to Midway Airport. The threat of a collapse also prompted Columbia College, which has two buildings directly across from the burned building, to cancel classes through Friday in those buildings.

"Safety is the number one concern until the fire is out and until it's deemed safe to travel through by train," said CTA spokeswoman Robin Ziegler. She said passengers could get around the problem area using a nearby line underground line.

Fire Department spokeswoman Eve Rodriguez said the fire appeared to have started in the building's basement and spread to its roof. One firefighter and another person were injured by the blaze but both were in good condition.

The 1887 building, once home to a steakhouse on the south side of Chicago's downtown Loop, was designed by the storied architectural firm of Louis Sullivan and Dankmar Adler. It was designated a city landmark in 1996.