The progressive growth of hidden cracks within railroad tracks, known as "rail cancer," is a major safety hazard in the railroad industry. Crosswise cracks that begin with tiny flaws inside the steel rails can grow into surface cracks and eventually cause the rails to break. In the effort to prevent derailments, service cars equipped with special sensors travel the nation's railroads, seeking out signs of rail cancer and other defects. But because the sensors require direct contact with the rails, the cars have to creep along at speeds averaging less than 15 miles per hour, forcing commercial train traffic in the area to shut down.

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