It is inevitable that after a train system has been operating for more than 100 years, it is bound to have experienced its share of accidents, wrecks, and mishaps. A number of factors contributed to the likelihood of accidents early on - on-sight signaling, tight headways, fragile wooden cars, mingling "L" cars with cars of other railways [the CNS&M and the CA&E, specifically] - and the likelihood of accidents has decreased markedly in modern times with the use of signaling systems. Still, many accidents have occurred.

The "L" does not have claim to the worst transportation disaster in Chicago (that dubious honor goes to the 1915 Eastland disaster in which 844 were killed when an excursion steamer capsized in the Chicago River) nor the worst train disaster (that happened in 1972 when two Illinois Central commuter trains collided at 27th Street, killing forty-five).

It is also worth noting that many "L"-related mishaps are not the fault of the CTA or their predecessors at all, but rather Mother Nature. A fair number of crippling problems have resulted from heavy snowfalls and other nature-related mishaps.

Listed below are those incidents which could truly be called disasters in some way or another (and the term is, of course, high subjective).

Train Accidents | Weather/Natural Disasters

Train Accidents

The 48th Avenue/Met Plunge - December 23, 1895
A sleeping motorman drove his single-car train off the end of the 48th Avenue stub-end terminal on the Garfield Park Line and onto the ground late one night.
The Rockwell Derailment - June 20, 1896
Only a week after switching from steam power to electric traction, a westbound motorcar on the Lake Street Elevated derailed near Rockwell Street on the morning of the 20th. The incident caused the Lake Street to revert back to steam power until the company could make certain modifications to the cars.
The Granville Rear-End Accident - November 24, 1936
A rear-end collision involving a North Shore Line interurban hitting the rear of an "L" train near the Granville station on the North Side destroyed the trailing wooden car, sending parts of the car body and injured passengers tumbling to the alley below in what was, for many years, the "L"'s worst accident.
The Wilson Collision - November 5, 1956
A deadly accident occurred when a 6000-series CTA train rammed the rear of a North Shore Line interurban stopped at Wilson to load and discharge passengers.
The Addison/Kennedy Collision - January 9, 1976
During the morning rush hour, two trains collided at Addison on the Kennedy Extension, in what was the most serious "L" accident up to that time (though unfortunately surpassed just 13 months later).
The Loop Crash - February 4, 1977
The accident that occurred in February 1977 at the corner of Lake and Wabash in the Loop lives on in many people's minds as the worst rapid transit accident in Chicago history. Four cars fell from the elevated structure after an eight-car Lake-Dan Ryan train rammed a six-car Ravenswood stopped east of the State/Lake station.
Various Other Incidents - various dates thru 2000
Besides the major incidents listed above, a number of other mishaps have occurred over the years. Listed here are some of these incidents, with dates and a brief description of each.

Weather/Natural Disasters

The Blizzard of '79
Although the Blizzard of 1967 occurred previously, the Blizzard of '79 was the first that crippled the "L" system in a serious way. With many cars and even sections of some lines out of service, the "L"'s service was severely disrupted for several weeks.
The Flood of '92
An accidental flood in the city's abandoned underground freight tunnel system flooded the CTA's State Street and Dearborn Street Subways downtown, not to mention many Loop basements!
The Blizzard of '99
In a repeat of the 1979 storm, a week of severe snow caused hundreds of rail cars to go out of service and crippled the "L" system for more than a week, closing sections of lines and causing long delays and disruptions in service.