Nobody was hurt but it shut down CTA Green and Orange line service for hours.
It gave the tourists on board a story and pictures of Chicago they will not soon forget.
There were 40 people on board the train who paid $100 to be on board.
The train, chartered by the Illinois Railway Museum, derailed just south of the Roosevelt Road station. Help arrived just moments after. The investigation is ongoing, but officials are looking at a rear wheel as the possible source of the accident.
"It was pretty wild," said passenger Eric Zabelny. "It just started bouncing around and stuff like that."
"I knew what was happening when it was happening," said passenger Ray Wert. "I just sat down and waited for the train to stop.)
It happened around 11:20 a.m. as the train headed northbound through a crossover on the tracks near 14th and Wabash.
The train, made up of eight cars from the 1960s and 1970s era, was chartered by museum for its fundraiser.
The McHenry County organization sold tickets hoping to purchase similar train cars.
Power to the train was cut while Chicago Fire Department personnel and others walked dozens of passengers along the elevated tracks to the nearby Roosevelt Road station.
The train derailed after it left Forest Park for a day-long trip through the city.
Passenger Harold Krewer, who uses a cane and needed the assistance of the fire department cherry picker, said all on board remained calm.
"We were all taking it in stride," Krewer said. "Nobody was upset, nobody was worried. It was just a diversion in our day."