Amtrak's Acela Express resumed limited service yesterday between Washington and New York with new brake discs replacing faulty parts that had forced the high-speed trains out of service in April.

Some passengers expecting to board Amtrak's slower Metroliner trains yesterday were put aboard Acelas instead. Two trains made two round trips between Washington and New York, carrying 875 passengers at a reduced Metroliner fare. Before the service was suspended, Acela trains made 15 daily round trips between the two cities.

The entire fleet will be back in operation in September, said Helene Gagnon, spokeswoman for the train's manufacturer, Canada's Bombardier Transportation.

Amtrak said that as each train is outfitted with the new brake discs and tested, it will be added to the service. But no rollout schedule is available, said Amtrak spokeswoman Marcie Golgoski.

Four Acela trains have been equipped with the new brake discs made by Westminster, Md.-based Knorr Brake Corp., but two are being held in reserve as backup, Gagnon said.

Service to Boston will resume once six trains are operating, said Cliff Black, spokesman for Amtrak.

Cracks in the train's brake discs prompted the suspension in April. The cause of the cracks is still being investigated.

Knorr had designed a new version of the brake discs a decade ago but was not called upon to manufacture them. The new parts use steel with a higher tensile strength.

Amtrak began to phase in its Acela trains in 2000, offering passengers a 25-minute shorter service at a premium. Tickets aboard the Acela cost $157 for one-way service between Washington and New York, while seats on the Metroliner cost $137.

Amtrak replaced its Acela fleet with Metroliner trains within two weeks after the brake problems were discovered. It has lost about $1 million a week while the Acelas have been out of service because it takes in lower fares for the Metroliner and has suffered a 5 percent decline in passengers in the past three months.

The brake problems come as Amtrak is fighting for federal funding.

Amtrak has requested $1.8 billion for next fiscal year, which begins Oct. 1. The House has agreed to provide $1.2 billion; the Senate will take up the funding issue this week.

"If we don't get the full amount that we requested, we would probably have to defer some capital projects that are designed to bring the railroad back to a state of good repair after years of deferred maintenance," Black said.

Last year, Congress approved $1.2 billion in subsidies for Amtrak.

"Despite the negative attention and publicity surrounding Acela, Congress shouldn't use the problems with Acela as an excuse to defund Amtrak, as the Bush administration proposed earlier this year," Sen. Thomas R. Carper (D-Del.) said in a written statement.