THAYER, Iowa (AP) _ Travelers ''flew from one side of the car to another'' when Amtrak's California Zephyr was sideswiped by a derailed freight car, injuring at least 40 people, one of them seriously, a passenger said.

The 13-car Zephyr, heading from Chicago to Oakland, Calif., with 307 passengers and 21 crewmembers, was struck by a car of a Burlington Northern freight train Thursday night, said Clifford Black, an Amtrak spokesman in Washington, D.C.

Most of the injuries to passengers and crew consisted of cuts from flying glass, although some suffered neck injuries and bruises.

Thayer is about 40 miles southwest of Des Moines.

''I heard a loud bang and I said, 'Oh, the train hit a car,''' said Leoline Hall, 63, of New York City, who was given a trip to San Francisco as a gift by her children. ''Then, when the lights went out, I heard another bang and the train stopped.''

One car of the 46-car freight train, which was hauling flour, derailed and tipped into the side of the Amtrak train, striking the second locomotive and all 13 cars of the passenger train, said Gary Telfer, a spokesman for Burlington Northern in Chicago.

''In this process, it broke most of the upper windows on the Superliner,'' he said. ''That, on Amtrak, is where most of the people are, especially in the coaches.''

The lower level contains mostly restrooms and baggage areas, he said.

The cause of the accident was unknown, Telfer said.

''We're still investigating,'' Telfer said today. ''Some of these things are very difficult to determine. There's so many things you have to check out and re-check. It's just like an airplane crash.''

Les Holland, a spokesman for the Iowa Department of Transportation, said the section of track had been inspected twice within the last year by DOT staffers and no defects were found. The checks were routine, he said.

An investigator from the National Transportation Safety Board's Denver office was sent to the scene, said spokesman Ira Furman in Washington.

Ed Elwell, a passenger from Bricktown, N.J., said he heard the freight train passing, then suddenly a big boom.

''Glass started flying everywhere,'' he said.

''I flew from one side of the car to another,'' said Edith Gallagher of Lansing, Ill., who was traveling with a group called Seniors On the Go. ''It looked like it was OK, then there was another jolt and I fell on top of another woman.''

Most of those injured were treated and released, but 11 remained hospitalized today. Gilbert Dunn, 80, of Davenport, was in serious condition at Iowa Methodist Medical Center in Des Moines with head injuries, officials said.

Most of the injured were taken to hospitals in Creston, Osceola, Mount Ayr and Winterset, where about a dozen remained early today, most in good condition.

Five Burlington Northern employees were operating the Amtrak train at the time of the accident because Burlington Northern owns the track in the area, Black said.

Most of the passengers were taken to Southwestern Community College in nearby Creston, where they were bedded down on wrestling mats in the gymnasium.

Officials had planned to take the passengers by bus to Lincoln, Neb. to another westbound train. But they said today the eastbound Zephyr would be stopped at Creston, emptied of its passengers and then sent back westward with passengers from the damaged train. Eastbound passengers would be taken to Chicago and other destinations by bus.

Dozens of volunteers from Creston came to the college and hauled food from the cafeteria to feed the passengers. Officials put out a call in the Creston area for pillows and blankets, and they were brought in by the carload.

''I'll tell you if you write this thing, you should write that these are wonderful people,'' said Owen Henricks of Neptune, N.J., a passenger.

Highway Patrolman R.H. Reid said a phone bank was being set up so passengers could contact relatives and assure them they were all right.

Amtrak's allowable speed in the region is 79 mph, Telfer said. He said he thought the freight train's maximum allowable speed would be 50 mph.

''I was on the side that hit,'' said Fern Pate of Littleton, Colo., who was traveling to Denver. ''It sounded like rocks hitting the train. I didn't think much of it until there was another jerk and we stopped.''