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. In others, trains operate using equipment designed, literally, in the horse-and-buggy era.

In Philadelphia, for example, commuters ride trains over rusty steel bridges, some of them dating back to the 19th century. The Southeastern Pennsylvania Transportation Authority - which operates subway, trolley, bus and commuter rail systems - is responsible for 346 bridges that are on average 80 years old. Officials said they may be forced to slow trains or even stop them from crossing one bridge that's 1,000 feet long and 90 feet above the ground if it deteriorates further, leaving stations on the other side without service.

A key power substation relies on electrical equipment manufactured in 1926. There's no hope for acquiring spare parts, so workers try to open the boxes housing the equipment as infrequently as possible to prevent damage from exposure to the environment.

"We're operating on a prayer on that line," Joseph Casey, the transportation authority's general manager, said in an interview. "If that fails, half of our commuter rail system would shut down." The system carries 125,000 passengers on weekdays.

The transportation authority doesn't have enough money to replace the bridges or outdated electrical equipment, Casey said. In recent years, the authority has spent $600 million on renovating elevated portions of a subway line that dates back to 1905, and $100 million to install a federally required rail safety system. Another $327 million was spent on new rail cars to replace 72 cars built in 1964. Some of the older cars - nearly 40 of which are still in service - are in such disrepair that passengers get soaked from leaks when it rains.

And yet passengers made 334 million trips on the transit system last year, the most in 22 years, despite a 9 percent fare hike. So far this year, ridership is up 3 percent, Casey said.

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.

BART plans to buy 775 new cars by 2023 at an estimated cost of $3.2 billion, but so far the agency has identified only about a third of the money - enough for the first phase of 200 cars, James Allison, a BART spokesman, said. Where will the rest come from? "We're working on that," he said.

Despite these problems, passengers are averaging 365,000 weekday trips this year, a record high, Allison said.

Peter Rogoff, administrator of the Federal Transit Administration, who toured a BART repair facility last week, said reducing the backlog of transit repair and replacement projects has been his agency's top priority for the past three years. The transit administration has awarded $1.53 billion for repair and replacement projects, and another $650 million is expected to be awarded this year. The Obama administration also has directed about $9 billion to transit projects through the economic stimulus act and a transportation grant program, although the grants weren't specifically aimed at the backlog.

"If we want the American public to be able to have a choice to avoid higher gas prices by using transit, then the transit service needs to be reliable and desirable," Rogoff said in an interview.

But because of lower tax revenues over the last several years, many state and local governments have cut back aid to transit systems, and many systems are increasing fares and cutting service because of tighter budgets, including agencies in Boston, Detroit and Pittsburgh.

Budget squeezes led Lorain County, Ohio, near Cleveland, and Clayton County, Ga., near Atlanta, to eliminate bus service all together. Transit supporters are trying to resurrect service in both communities.

"Even though we've increased federal investment in this area, the crunch on state and local municipal dollars, as well as the falloff of tax dollars during the recession, has really impacted their ability to bite off the bigger challenges," Rogoff said.

The Roaring Fork Transportation Authority in Colorado, one of the nation's largest rural bus services, is in the process of upgrading to a "bus rapid transit system" that will use intelligent technology to shorten commute times, allow passengers to get real-time information on the status of buses using their smartphones, and provide amenities like WiFi, said Dan Blakenship, the authority's chief executive officer.

The authority provides bus service to mountain towns and ski resorts in the Roaring Fork Valley, including Aspen and Glenwood Springs. Yet the agency has been unable to afford to build garages for most of its buses. With temperatures often dropping into the teens at night, workers have to start up buses at 2 a.m. each morning in order to warm them up before the first get out on the road two hours later. It wastes fuel, cost money and is hard on bus engines, Blakenship said.

And the escalating price of fuel makes matters worse, he said. "Every time we get some wind in our sails, we get hit with one of these diesel fuel price spikes. It's not easy to compensate for that."