Amtrak plans to stop carrying mail in October so it can focus on its core business of transporting passengers.
"Amtrak's management decision to transport mail in box cars and passenger trains has been disruptive to passenger train operations," said Amtrak spokesman Cliff Black.
The interference includes delays from coupling and uncoupling freight rail cars to passengers trains.
"The profit margin is small and we feel that making these changes will improve our bottom line, make the trains more efficient," Black said.
Amtrak informed the United States Postal Service (USPS) of its decision last week.
The national passenger railroad carries mostly bulk mail under a $60 million per year contract with the USPS, reported The Washington Times.
Transporting of bulk mail will likely shift to freight railroads, long-haul trucks and airline service, according to the USPS.