Hidden cracks likely caused Swedish high-speed train wheel failure, expert says

Undetected cracks beneath the surface likely caused a wheel to fracture on one of Sweden’s SJ high-speed trains, forcing the entire fleet to be taken out of service, according to a railway mechanics expert.

Anders Ekberg, professor of railway mechanics at Chalmers University, told public broadcaster SVT Nyheter that the incident—where a piece of wheel broke off after crew reported unusual noises—points to a rare type of subsurface crack formation. Such defects are difficult to spot without ultrasonic testing and can grow severely before detection.

“It sounds like a fatigue crack that started beneath the surface. Once it grows long enough, it reaches the surface and causes a chunk to break off,” Ekberg said. “If that’s the case, you’re left with a seriously damaged wheel.”

Thousands of passengers have been affected after SJ suspended its high-speed services between Stockholm–Gothenburg and Stockholm–Malmö following Sunday’s incident. Subsurface cracks pose a higher risk because they can expand unseen, potentially damaging both the wheel and track infrastructure.

“There’s definitely a derailment risk,” Ekberg said, though he noted he had never heard of an actual derailment caused by this specific type of crack. The technical term for the damage is subsurface-initiated rolling contact fatigue, occurring when internal cracks form in wheels due to high-pressure contact with rails.

Unlike surface cracks—commonly caused by friction and routinely fixed by wheel reprofiling—subsurface defects require ultrasonic inspection to detect. Carl-William Palmqvist, a railway researcher at Lund University, warned that such cracks can lead to sudden wheel failure during operation, risking derailment or serious accidents.

SJ has declined to confirm the crack type but is conducting an urgent inspection of all wheels. “Right now, we’re checking every wheel to ensure we don’t find the same fault,” said Peter Kraméus, SJ press officer. A deeper investigation with the wheel supplier will follow, expected to take several weeks.