Amtrak President David Gunn acknowledged on a Washington, D.C., radio program Wednesday that a Madrid-like explosion could occur in the United States, but he said Amtrak has not received any specific threats.

"I don't think there's any point in pretending there isn't some risk," said Gunn. "If people are trying to do damage to you, there is no way you are ever going to be in a situation where you can be 100% certain that something can't be done."

Gunn said Amtrak commuters are as safe as the rest of the country but admits it would be impossible to put in place a screening process like the ones airlines use. He indicated that screening passengers and luggage along commuter rail lines could destroy the service.

Amtrak is seeking $110 million to improve security, particularly in its tunnels.