To reduce travel times and accommodate growing ridership, Amtrak said Thursday that it plans to replace existing Acela Express trains with new, faster trains on the Northeast Corridor.

But don't buy a ticket just yet. The first of the new trains isn't likely to arrive until at least 2017.

Amtrak currently operates 20 Acela Express trains, the fastest in its fleet. The trains can travel up to 150 m.p.h., though because of track and signal restrictions, they average only 83 m.p.h. between Washington and New York and only 72 m.p.h. between New York and Boston.

Amtrak president Joe Boardman told a congressional committee Thursday that Amtrak is scrapping plans to buy 40 new cars, to add two cars to each of the existing six-car Acela trains, which began operating in 2000.

Instead, Amtrak will issue a "request for information" in early 2013 for proposals on how to replace all of the Acelas gradually with entirely new, faster trains. Later in the year, Amtrak will seek bids for such trains.

"Moving directly to new high-speed train sets is the best option to create more seating capacity, permit higher speeds, and maximize customer comfort, all while improving equipment reliability and reducing operating costs," Boardman said.

The cost will be determined by what is learned from the proposals next year, Amtrak spokesman Steve Kulm said. Amtrak hopes to receive the first of the new trains in five to seven years, he added.

At the same time, Amtrak is spending $450 million to improve the Northeast Corridor main line between Trenton and New Brunswick, N.J. That will increase maximum speed for Acela trains on that 24-mile stretch from the current 135 m.p.h. to 160 m.p.h. by 2017.

Amtrak's long-term goal is to build a dedicated route for 220-m.p.h. trains as part of an upgraded 438-mile Northeast Corridor, including new tunnels under the Hudson River.

That would allow 37-minute trips between Philadelphia and New York, 94-minute trips between New York and Washington, and 2½-hour trips between Philadelphia and Boston.

Amtrak has estimated that it would cost $151 billion and take until 2040 to complete.

Some congressional leaders said Thursday that was too much money and too much time.

House Transportation Committee Chairman John Mica (R., Fla.) said that, despite recent progress, "Amtrak's $151 billion, 30-year plan is simply too much money and too long, and we must bring in the private sector if [Northeast Corridor] high-speed rail is going to become a reality."

Boardman said Amtrak expects private funding to be part of the solution, but he noted that the experience of other countries in building high-speed rail systems shows that "only after the public sector has allocated significant funding and committed itself to a project of this magnitude is the private sector willing to enter the deal and deliver value for money."