William Crosbie, the railroad's chief operating officer, will serve as acting president and CEO. Amtrak said Kummant will help with a transition to new leadership.
Donna McLean, chairwoman of Amtrak's board, thanked Kummant for his service in a statement released Friday.
"During the past two years, the company experienced exciting economic times, strong ridership and revenue growth, solid profit and loss performance, and the advancement of the Amtrak brand overall," McLean said.
Kummant, hired as president and CEO in 2006, also is credited with overseeing the completion of labor agreements with all Amtrak's union employees. He took over for David Gunn, who was fired in 2005, a year in which Amtrak had to suspend all high-speed Acela service after discovering cracks in some brakes.
Kummant previously served as an executive with Omaha, Neb.-based railroad operator Union Pacific Corp.
Amtrak's board said it will continue to try to increase ridership as fuel costs and other economic concerns send more commuters to trains and buses.
"Current economic conditions highlight the need for us to continue finding ways to drive quality and customer service across the system," said board vice chairman Hunter Biden, the son of Vice President-elect Joe Biden.