Andy Byford, New York City’s new transit chief, believes he can slash decades from the timeline for modernizing the subway system. But he admits, his aggressive plan would cost billions of dollars and years of rider pain in the form of weekend outages.

“This plan will need money, it will need time, it will need political will, and it will need the forbearance of New Yorkers,” Mr. Byford said in an interview with The Wall Street Journal. “It will not be without inconvenience.”

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