The unprecedented safety shutdown of Washington’s subway for a day in March laid bare the deteriorating conditions that created a crisis.

But the Metro, which opened four decades ago and shaped the Washington region, is not the only transit system in the Northeastern United States showing its age.

The subways in New York City and Boston, both more than half a century older than the system in Washington, need billions of dollars to replace aging infrastructure and to meet rising demand. All three cities, home to most of the nation’s busiest subways, are growing and attracting young people, who prefer mass transit but have become frustrated by delays.

As transit officials in Boston, New York and Washington focus on improving the subways, their efforts are being closely watched by planners and business groups who fear economic growth in the region could slow if the systems cannot keep up.