BOSTON — Urban planners have long considered public transportation the best remedy for traffic congestion, but many of the nation’s largest mass-transit systems simply aren’t up to the task.

They lack the money to keep up with basic maintenance, let alone modernize and expand to attract more riders and take pressure off the highways connecting growing cities and suburbs.

Consider Boston, where the oldest American subway system began operating in 1897. During a winter of record-setting snow, the Massachusetts Bay Transportation Authority was often paralyzed, its antiquated equipment overwhelmed by the elements.

By late February, “the T,” as locals call it, had less than half of its fleet available. Service was not fully restored until the end of March.

At the height of one storm, police helped ferry doctors and nurses to hospitals so emergency rooms could stay open. Schools closed. Businesses suffered, along with countless workers, especially hourly wage employees who couldn’t get to their jobs.

Among those was Patricia Varon’s disabled son.

“It was a loss of income for him,” she recalled. “He waited for hours, and the train never came.”

What happened in Boston over the winter was extreme, but transportation experts and policymakers say it’s emblematic of chronic underinvestment in urban transit systems at a time when demand is growing in many areas.

“Boston is a case in point that we’re undershooting the target, and we’ve been doing it year after year after year after year,” U.S. Transportation Secretary Anthony Foxx said in a recent interview.

Under ideal circumstances, public transit systems would extend their reach, offering more trains, buses and ferries, longer hours and new routes serving a wider geographical area. Newer systems in cities such as Los Angeles and Denver have plans to do just that.

But the so-called “legacy” systems serving Boston, New York, Philadelphia, Chicago, San Francisco and Washington face a dilemma: Should they use limited money to shore up their creaky infrastructure or stretch it to expand service as populations grow?

In Boston, some critics of the T say the agency dug itself in a hole by expanding even as its core infrastructure deteriorated, leaving the nation’s fifth-largest transit system heavily in debt.

The Pioneer Institute, a nonprofit research group in Massachusetts, found the T added more commuter rail miles than any comparable U.S. system in the past 25 years, but with questionable results. It found that ridership on the 394-mile commuter rail network actually dropped between 2003 and 2013.

A task force assembled by Gov. Charlie Baker after the winter storms called for a moratorium on most expansion, a stance criticized by some transit advocates.