WASHINGTON — Driven by high gas prices and an uncertain economy, Americans are turning to trains and buses to get around in greater numbers than ever before. But the aging transit systems they’re riding face an $80 billion maintenance backlog that jeopardizes service just when it’s most in demand.

The boost in ridership comes as pain at the gas pump and the sluggish economic recovery combine with a migration of young adults to cities and new technology that makes transit faster and friendlier than in the past. The number of transit trips over a 12-month period will likely set a new record later this month or next, say Federal Transit Administration officials. The current peak is 10.3 billion trips over a year, set in December 2008.

But decades of deferred repairs and modernization projects also have many transit agencies scrambling to keep trains and buses in operation. The transit administration estimated in 2010 that it would take $78 billion to get transit systems into shape, and officials say the backlog has grown since then. In some places, workers search the Internet for spare parts that are no longer manufactured.

San Francisco’s subway system, Bay Area Rapid Transit, faces many similar problems. Opened in 1972, BART was at that time the most automated subway system in the nation. But circuit boards and other electronic components for 449 original train cars — out of the system’s total of 669 cars — are now 40 years old, no longer manufactured and often impossible to replace.

BART employees regularly scour eBay and other websites in search of after-market dealers who might stock the parts, said Tamar Allen, manager of BART’s mechanical operations. When they find a dealer, they buy every useable part until “the well runs dry,” she said.

And that’s still not enough. Some cars have been cannibalized for parts in order to keep other cars working. Cars whose parts have been removed are still in use, but only when they can be sandwiched between other cars, Allen said. In some cases, employees have re-engineered parts when no replacements could be found, but it’s a difficult process because there is no margin for error, she said.