Rail transport experts say heat is the chief cause of cracked wheels on rail cars. Regular inspections can often detect such cracks before they lead to a catastrophic failure, but not always.
"When a wheel fails, a portion of the truck the wheel is riding on can drop down ... getting under the train and causing a derailment," said Colon R. Fulk, a railroad operations consultant with Railex Corp. "Where this happens on a curve, an overpass or trestle, you can imagine the outcome."
The Maryland Transit Administration found a crack in a wheel of a light rail car after the April 23 derailment of an out-of-service car. No one was hurt, and no other cracks have been found, but riders have experienced long delays and crowded trains as the agency has pulled cars out of service for inspection.
Wheel failures on heavy rail and commuter rail cars and locomotives have caused 4,500 accidents since 1975, with 18 deaths and 284 injuries, according to the Federal Railroad Administration.
A broken wheel was blamed for the 1994 derailment of a Barnum & Bailey circus train in Lakeland, Fla. Two circus workers were killed and 15 were injured.
Light rail transit systems use lighter cars and travel at lower speeds.
"I am not aware of any accidents in the light rail world due to cracks," said Tom Peacock, director of operations and technical services at the American Public Transportation Association, which represents public transit operators in Washington.
The cracks generally develop because of thermal stresses, he said: "They brake a lot. That causes heat, and the heat builds up in the wheel. And if the wheel doesn't have time to cool between stations, a crack can develop."
Transit systems with frequent stops need to understand their heat loads, buy wheels manufactured with higher heat tolerances if needed (which can be more costly) and follow up with frequent inspections, he said.
Some cracks are visible to the trained eye, forming radially near the rim where the brakes are applied. Trains can run for some time with cracks, Fulk said, but "regular ... observation" by maintenance workers and train crews is needed to spot them before they lead to failures.
Some cracks begin invisibly inside the wheels. And, "as transit systems grow, it becomes impractical for inspectors to wander through the yard and look for defects," said James Skaggs, director of railway equipment at International Electronic Machines Corp. in Albany, N.Y.
In those cases, he said, ultrasonic systems might be needed to probe the wheels for defects.