It took just a few weeks for a small team of university engineers working pro bono to solve a problem that has vexed the nation’s largest transit agency and a global consulting firm for years.

In 26 days, starting Dec. 7 through New Year’s Day, six engineers from Columbia and Cornell universities developed a workaround to avoid the full shutdown of a busy section of New York City’s L-train subway line that has been planned—and dreaded—for more than two years.

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