Federal officials have stepped up surprise inspections of low-fare bus companies in the Washington-to-Boston corridor amid concerns by federal and state regulators that some curbside operators are unsafe and under-regulated.

A late October inspection sweep of 400 buses by a task force of federal, state and local authorities turned up more than 500 safety-related violations, according to the Federal Motor Carrier Safety Administration, which regulates interstate bus and truck companies. The violations included speeding, a broken horn, and inadequate brakes and windshield wipers.

Following the inspections, the FMCSA ordered 56 buses and 13 drivers out of service, agency Administrator Annette M. Sandberg said yesterday.

Most of the buses inspected were operated by budget carriers that have become increasingly popular, picking up passengers at designated curbsides and offering round-trip fares as low as $35 between Washington and New York -- below that charged by major carriers such as Greyhound. The budget carriers are commonly known as Chinatown buses because they once primarily served Asian communities in the Northeast.

With the holiday travel rush underway, the agency has targeted specific companies for detailed reviews in early December, Sandberg said.

"My concerns are with operators who operate on the margins," she said. "Whenever somebody is operating on a very low margin . . . the first thing they cut is safety . . . whether it's safety management or maintaining the equipment or making sure they are doing drug or alcohol testing on their drivers . . . or carrying the proper levels of insurance."

Separately, the Justice Department says it has launched two investigations into whether discount carriers are complying with provisions of the Americans with Disabilities Act, department spokesman Eric Holland said. Sandberg said she has relayed to the department reports of budget operators that do not have wheelchair lifts on their buses.

The new government scrutiny follows two fires involving low-cost carriers on the New York-Boston route this year and concerns raised by Sen. Charles E. Schumer (D-N.Y.) that some discount carriers are being allowed to operate with "egregiously low" federal safety ratings.

A review by The Washington Post found that three companies offering service from the District to New York this year received low FMCSA safety ratings, including one, Tomorrow Travel & Tour Inc., that was ordered out of service last summer but kept operating.

"I am very concerned that because of lax safety precautions and insufficient oversight more incidents could happen, putting hundreds of thousands of riders in danger," Schumer wrote Sandberg on Aug. 28.

Sandberg agreed more needs to be done.

"I can tell you, being a former law enforcement officer: You try to get to crime or safety before the problem exists," said Sandberg, former chief of the Washington State Patrol. "But oftentimes, you are reacting at the back end."

Overall, bus travel is relatively safe, with about half the fatality rate of automobile travel. But concerns about the industry have peaked recently, with the spreading popularity of budget carriers and a handful of high-profile incidents. Most recently, a bus fire in Texas killed 23 hurricane evacuees leaving a nursing home.

The FMCSA formed a task force in 2003 to investigate low-fare carriers in the Northeast. In a speech to the American Bus Association last year, Sandberg reported: "Our investigations revealed a complex web of business relationships among these low-fare operators. Dozens of motor carriers are interwoven and share their business in a way that makes it challenging to determine who is responsible for their operations."

Gladys Cole, an FMCSA spokeswoman, said one enforcement problem is that bus carriers ordered out of service can start up again after forming a new company and paying a $300 registration fee.

"You can go online and apply for authority [to operate], pay your $300, and you're back in business," she said. "And when the heat gets on you, you go out of business -- or the agency puts you out of business -- you pay $300 and you start up again."

Among companies that serve the Washington region, Tomorrow Travel & Tour, which operated a bus line called Dragon Coach, was ordered to suspend service on Aug. 8 after it failed to pay a $4,400 fine. The fine was assessed because the company did not register with the FMCSA and did not have a required drug- and alcohol-testing program for its drivers, according to Robert W. Miller, a special assistant at the agency. The FMCSA has tacked on an $8,800 fine -- also overdue -- because the company continued to operate after being ordered to shut down, Miller said.

A company called Dragon Expressway & Travel Inc. is now selling bus tickets under the Dragon Coach name and is using the same address and phone number Tomorrow Travel & Tour used.

A woman who answered the phone at the bus company office said one bus was scheduled to go from Washington to New York yesterday, leaving from 14th and L streets NW, with seven scheduled for today. The woman declined to give her name and referred questions about the company's ownership to a manager. The manager did not return a call.

Schumer's letter to Sandberg in August was written 12 days after a bus operated by Boston-based Fung Wah Bus Transportation Inc. burst into flames on a Connecticut highway. The 45 passengers escaped before the fire started. In March, a coach run by a bus line called Travel Pack caught fire. Those passengers also escaped unharmed.

On the Washington-New York route alone, about 10 low-cost carriers have emerged in recent years to challenge the traditional carriers, Greyhound and Peter Pan. The budget buses typically charge $35 round-trip on that route -- together offering more than 50 departures on peak travel days, such as today. Greyhound and Peter Pan have dropped their fares. Greyhound trumpets a $20 one-way fare for tickets bought on its Web site.

The discount companies vigorously compete for business on Web sites that provide e-ticketing and a plethora of reasons why their service should be chosen. "Customers' satisfaction is our No. 1 priority," says Philadelphia-based New Century Travel.

Some budget travelers like the buses just fine.

"They're really clean, they show movies, they have bathrooms," said Margot Zengotita, in town doing research at the Library of Congress, as she waited for the day's Dragon Coach bus to New York.

Special contributor Mark Chediak, staff writers Ellen McCarthy and Lena Sun, and researcher Richard Drezen contributed to this report.

District passengers Euri Lorenzo, left, and Isham Randolph have their reservations checked as they wait for their bus to depart for New York.