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Just as the East Coast developed into a string of megacities linked by Amtrak, California is evolving into its own megalopolis.

The rail line, which serves Metrolink’s commuter trains and Amtrak’s Pacific Surfliner, was closed in April to begin construction.

Upgrading the Northeast Corridor, the train tracks that run from Boston to Washington by way of New York, Philadelphia, and Baltimore, with spurs to neighboring cities, has been the longtime dream of the region’s leaders and planners at Amtrak, which runs the intercity trains.

A decade prior, Amtrak had drawn up a plan that would cut the train time between Boston and Washington from six and a half hours to three and a half hours.

In a new Transit Costs Project report published Friday, they and their co-authors sketch out a plan for Amtrak to get the Boston-to-Washington travel time under four hours for $17 billion.