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It was springtime in the Sierra, Shelley Fisher Fishkin said. There was no snow on the ground.

And yet, Ms. Fishkin, an English professor at Stanford University, recalled, the guide she and colleagues were touring with slipped on a patch of black ice in a tunnel and broke a shoulder.

“It just was an emblem,” she said. “If that’s what the tunnels were like in April, how treacherous they would be in the winter.”

Ms. Fishkin, one of the directors of the Chinese Railroad Workers in North America Project, had for years been researching the experiences of those who blasted through mountains and laid track in the desert to complete the Transcontinental Railroad.