Originally designed in 1903 to hold 650 passengers, the ship was retrofitted in 1913 with the aim of increasing its capacity to 2,500. It had a history of listing due to its top-heavy design, dating back to its first year.
On the morning of July 24, 1915, some 7,300 employees of Western Electric Company gathered at the dock between LaSalle and Clark streets to be carried out to an annual company picnic in Michigan City, Indiana by the Eastland and four other steamers. Some reports suggest that more than 2,500 people boarded the Eastland; there are also stories that the crowd tipped the boat off balance by gathering on the port side of the boat to pose for a photographer. What is known as fact is that engineer Joseph Erikson opened one of the ballast tanks, releasing water within the boat meant to stabilize the ship. The Eastland capsized moments after leaving the dock.
Ironically, the weight of the lifeboats added to the Eastland as a result of the U.S. Seaman's Act, passed after the Titanic disaster three years earlier, contributed to the instability of the ship.
A nearby vessel, the Kenosha, came alongside the hull to allow those stranded on the capsized vessel to leap to safety. But hundreds were trapped below decks, some crushed by pianos, bookcases, and other furniture. Many of the dead would be taken to the 2nd Regiment Armory (there are stories that they continue to haunt the building, later used as Harpo Studios by the Oprah Winfrey Show, but I've never heard that any of them made an on-camera appearance).
The Eastland itself would be salvaged, renamed The Wilmette, and put into military service as a gunboat in World War I. It was scrapped after World War II.
The last known survivor of the capsizing was Marion Eicholz, who died on November 24, 2014, at the age of 102.
Chicago Tribune accounts, photographs, and a couple John T. McCutcheon cartoons are on the Chicagology page here.