Amtrak will now seek bids next month from manufacturers for up to 28 new Acela train sets that can begin to roll into service between Boston and Washington by late 2018, Yachmetz said Friday.
The train sets, with a power car at each end and six passenger cars, are expected to cost about $50 million each. With additional seating capacity in the new trains, each Acela could carry 450 or more passengers, compared with the current 304.
Because Acela operates at a profit, financing the new trains won't depend on congressional appropriations.
The Obama administration had urged joint procurement for California and Amtrak as a way to reduce costs and boost train manufacturing in the United States.
High-speed train manufacturers, though, had long expressed doubts about the feasibility of building a common train that could be modified to meet the needs of California and Amtrak.
"It became clear to us that we would get suboptimal solutions as these compromises took place," Yachmetz said, citing the prospect of "serious risks as to schedule and costs."
Amtrak envisions buying 28 new train sets to add to its current fleet and to replace the 20 existing Acela train sets by the early 2020s.
Amtrak's new Acela trains are expected to be sleeker, more reliable and more comfortable, but trip times are not slated to be shorter.
The Northeast Corridor, with its current configuration and equipment, can't handle trains faster than 160 m.p.h, Amtrak says.
Amtrak's existing Acela trains now travel up to 150 m.p.h. on a short stretch in New England, but the top speed between New York and Washington is 135.
And the train's average speed is considerably lower: On the 319-mile trip between Philadelphia and Boston, including stops, Acela averages about 64 m.p.h.
If manufacturers can come up with designs that permit faster trips on the Northeast Corridor, "that would receive significant favorable attention" from Amtrak, Yachmetz said Friday.
Like the existing Acelas, the new Amtrak trains are to be built with the ability to tilt on the Northeast Corridor's tight curves. Most modern European and Asian high-speed trains, traveling on wide curves, are not built to tilt.
With existing Acela service running near capacity on most trips between New York and Washington, Amtrak wants to increase service to every 30 minutes during peak periods and replace increasingly balky Acela trains with more reliable models.
"Amtrak needs more high-speed trains, with more seats per train, to provide more service for the growing ridership, mobility and economic needs of the Northeast region," Amtrak said in a statement Friday.
Amtrak in 2012 outlined a $151 billion vision for high-speed travel on a rebuilt Northeast Corridor, with 220-m.p.h. trains between New York and Washington by about 2030 and between New York and Boston by 2040. That plan envisioned 37-minute trips between Philadelphia and New York.
That is still Amtrak's long-range goal, chief executive Joseph Boardman has said, but the focus now is on incremental upgrades.
Amtrak is currently rebuilding a 23-mile stretch of track in Central New Jersey that will permit 160 m.p.h. travel. Boardman has said that $450 million project is part of a gradual approach to upgrading the corridor, with limited money from Congress and no multiyear funding.