Record numbers of riders are piling into public transit systems around major cities, motivated by high gas prices, worries about the costs of driving and a concern about the environment. American Public Transportation Association president William Millar told USA Today last year that usage data was higher in 2007 than it had been in 50 years. Last year APTA calculated a "modern record" of 10.7 billion trips. First-quarter data of 2 .6 billion trips exceeded expectations, even though gas prices declined and unemployment continued to rise. Ridership may double over the next 20 years.

"Now is the time to make the critical investments that the nation requires in its public transportation systems, highway infrastructure and high speed rail corridors," said Millar in a statement supporting a proposed transportation bill that would give surface transportation $450 billion.

But as their popularity surges, their funding needs to become more acute. A recent survey of transit agencies indicates that 80 percent expect their funding to be flat or declining this year, according to the APTA. A spokesman noted that nine out 10 transit agencies were forced to decide between raising fares or cutting service. Another problem these agencies are facing is debt.

New York's Metropolitan Transportation Authority is a case in point. MTA is a vital part of New York City's economy, transporting about four of five rush-hour commuters in and out of the Big Apple's business districts. It's the largest system in the U.S., carrying about 33 percent of the nation's public-transit passengers. The system, according to New York State, has a debt service of about $1.5 billion, which may more than double over the next few years.

"MTA is facing extreme financial pressure as a result of near- to-medium-term operating and capital hurdles given the rapid decline in operating subsidies, primarily mortgage recording tax revenues and other regional tax sources due to the downturn in regional housing prices and the current national and regional economic downturn," according to Fitch Ratings.

New York City has a vastly larger and less technical system than Washington, said Gene Russianoff, a staff attorney with the riders' advocacy group Straphangers Campaign. There has not been a fatality on the region's transit system since 1979. State officials recently approved two years of funding as part of a $23 billion bailout bill.

Kate Slevin, executive director of the Tri-State Transportation Campaign, a watchdog group for the region's transportation system, told DailyFinance she doesn't believe that maintenance has been short-changed because of the MTA's money woes. "We have definitely underfunded our infrastructure needs," she said. "The transit system in our region is still safe... [It's] vastly improved over where it was in the early 1980s. It is one of the best urban economic revitalization stories."

Though the system is safe, challenges remain, and finding enough funding remains an issue.