Park spokeswoman Denise Germann said the park understands the foundations concerns and is working with a consultant to address them.
"We love the red buses, too," she said. "We think they are iconic and we have no intention of not keeping the red buses on the road. There is no expectation that we will phase out the fleet."
The prospectus expects the red bus fleet to fail at some point during the term of the contract.
Hagen said the park has not said why it expects the fleet to fail. He believes the buses are on track to continue running well for years after the complete renovation just over a decade ago with the help of a $6.5 million donation from Ford Motor Co.
He said he'd like to see the bid process suspended to allow time for public comment on the future of the red buses.
Germann said by the end of the contract, the 17-passenger buses will have been on the roads for nearly 30 years.
"They're being driven every day, very intensely, for the summer period," she said.
She said the replacement buses would have to be something similar to the red buses that offer the same tourist experience. They would also have to be wheelchair accessible.
Any decommissioned buses would be returned to the National Park Service, Germann said.