Amtrak says the new trains will enter service in 2021.

Quoting from their published announcement (excerpts):

"At Amtrak we know the future of the Northeast Corridor (NEC) depends on the investments we make today, which is why we are excited to announce the upcoming arrival of the next-generation of high-speed rail. The new trainsets will replace the current Acela Express equipment and begin service in 2021. As part of this multifaceted modernization program, Amtrak is also investing in the infrastructure needed to improve your customer experience onboard the train and in major NEC stations including Washington Union Station and Moynihan Station New York."

"Just a few of the new amenities include:

• Approximately one-third more passenger seating, while preserving the spacious, high-end comfort found onboard today
• Modern interior design
• Improved Wi-Fi access and quality
• Personal outlets, USB ports and adjustable reading lights at every seat
• Enhanced food service options
• Exceed the ADA minimum accessibility requirements"

"...Acela Express service will be offered every half-hour between Washington, D.C. and New York City during peak times, and every hour between New York City and Boston throughout the day..."