Amtrak announced yesterday that it will complete restoration of service for its Acela Express line on Monday, five months after the troubled railroad suspended the trains for having a faulty brake component.

In April a safety inspector with the Federal Railroad Administration noticed hairline cracks in disk-brake rotors underneath an Acela train. A resulting investigation turned up hairline cracks in 300 of 1,440 brake disks in the 20-train fleet, prompting Amtrak to shut down the high-speed service that connects Boston, New York and Washington.

The troubled railroad has been gradually returning the Acela train to service over the past few months as it replaced the faulty brake part with a newly designed component and put in place a new set of safety inspection procedures.

On Monday, the railroad will add four round trips per weekday between Boston and New York, restoring the service to eight round trips a day, the same schedule as before the brake problem was discovered. The railroad operates 14 round trips between New York and Washington each weekday.

An official cause for the cracks in the components was never given, Amtrak spokesman R. Clifford Black IV said yesterday.

"That's still under investigation by the manufacturers," he said. The original disk-brake rotors were designed to last for more than a million miles of service; the cracked parts had been in use for only about half that amount.

If the problem had gone undiscovered, Amtrak's inspector general said, the cracks could have caused a catastrophic accident. Before Congress in May, Fred E. Weiderhold Jr. told of a rotor falling apart in the hands of an engineer when it was taken off the axle.

Acela is the railroad's most profitable line. Amtrak Vice President of Operations William L. Crosbie told Congress in May that the railroad would lose $1 million every week the Acela was out of service.

The railroad tried to make up for the reduced train schedule by running its lower-cost trains more frequently. Amtrak said yesterday that it retained most of its Acela customers over the summer but that it had lost some to other modes of travel.

The financial impact of the Acela shutdown appears to be less severe than originally forecast. Just after the Acela was sidelined, the railroad projected that it would end the fiscal year with $35 million cash on hand. Amtrak now projects that it will end its fiscal year, which concludes this month, with $120 million in cash available, Black said.

"We did lose revenue; I can't quantify it," said Black.

So far, Amtrak has not had to pay for replacing the faulty brake components. The Acela fleet is maintained by the consortium of companies that was contracted to build it: Bombardier Inc. of Montreal and Alstom, a French company. Amtrak is scheduled to take over maintenance of the Acela fleet next year.

On Sept. 9, Amtrak announced fare increases that would greatly reduce discounts for riders who use monthly Smart Passes. The fare increases were set to start this Tuesday but have been postponed indefinitely as Amtrak officials meet with public officials to explain the hike.