Amtrak restarted the Acela service with two daily roundtrips, Monday through Friday, between New York and Washington.
The remaining 18 Acela trains will be put back in service once they are equipped with the new brakes, Amtrak spokeswoman Tracy Connell said. No date was announced for resumption of Acela service to Boston.
Montreal-based Bombardier, Inc., which makes the Acela trains, said Monday its target date for equipping the entire Acela fleet with the new brakes is September. Bombardier is still investigating what caused the millimeter-size cracks in 317 of the Acela's 1,440 disc brake rotors. The cracks were found on all 20 Acela trains.
The cracks were noticed April 14, when Federal Railroad Administration Safety Specialist Rich Thomas found them during a routine inspection after a high-speed run to test whether Amtrak could speed up the Acela trains slightly on curves in New Jersey between Trenton and Newark.
Bill Crosbie, Amtrak's senior vice president, said 95 percent of Northeast Corridor passengers continued to use Amtrak trains during the Acela disruption. But the railroad added that its business in the Northeast corridor has fallen by 5 percent since the Acela was taken out of service.
"Acela Express is enormously popular with our passengers, and we're very glad to begin rolling these trains back into service this week," Crosbie said.
Amtrak has been running older and slower trains between Washington and Boston at lower fares to make up for the lost Acela service.
The new brake discs were designed jointly by Bombardier, Alstom SA of France, Knorr Brake Corp. of Westminster, Md. and Faiveley Transport of France. Alstom also makes the Acela trains.