Built-up tar in a ventilation shaft at the Old Post Office was ignited by a passing or idling train, causing Friday's roof fire, a Chicago Fire spokesman said.
The fire caused no injuries, but sent five fire trucks to the building, at 433 W. Van Buren St.
Firefighters attempted to access the roof using stairs to the roof and found flames on the upper floors, where the was no water and firefighters used hand extinguishers to put out the flames.
The fire department said the building’s "stand pipes" were not working and a tower ladder had to bring hose to the roof to contain the fire.
Earlier this month, the owners of the building, International Property Developers, were criticized for allowing the building to fall into a state of disrepair.
Several exhaust fans in the building, meant to remove soot and smoke generated by the trains operating on the Union Station tracks below, were found to be not working. The developers vowed to make repairs.
The Eisenhower Expressway -- Interstate 290 -- cuts through the building as it becomes Congress Parkway. The United States Postal Service vacated the building in 1997 for a newer facility.