OTTUMWA, Iowa (AP) _ The switching mechanism used to reroute an Amtrak train onto a side track where a Burlington Northern construction crane was parked worked perfectly when tested by rail officials, the chief investigator said Tuesday.

Officials from the National Transportation Safety Board are investigating Monday's crash, which jolted the 16-unit California Zephyr to a stop and knocked all but three of its cars off the track near Russell, injuring nearly half the estimated 270 people on board.

Officials from Burlington Northern, Amtrak and the NTSB were trying to determine if it was human error or mechanical failure that allowed the Zephyr to be switched onto the same track as the construction crane.

At a 13-minute media briefing Tuesday night, NTSB chief investigator Ed Dobranetski said the switch worked as it should have during a test earlier in the day.

However, Dobranetski and another NTSB official, Jim Kolstad, said they do not want to draw any conclusions from the test yet.

''Anything at this point would be purely speculative,'' Dobranetski said. Meanwhile, an engineer in the head locomotive said Tuesday that the startled crew of the Zephyr ''didn't have a chance'' to stop once they spotted an open switch routing the speeding passenger train onto the side track.

''All three of us saw it at the same time,'' said Robert Campbell, a 37- year-old Burlington Northern railroad veteran who was riding ''pilot'' for the Amtrak crew when the accident occurred shortly before noon Monday.

Investigators were centering their attention on a Burlington Northern work crew that had left for lunch moments before the eastbound train barreled into a crane, used for re-laying track.

''The only thing we know so far is that the switch was not aligned properly,'' Burlington Northern spokeswoman Yvette Brown said in Chicago. ''It has not been determined if it was human error or if it was a mechanical problem.''

She said the switch is a manual device, and train crews use heavy keys to gain access.

''If the spring isn't working properly, it can flip back without anybody knowing,'' she said. ''I'm not sure if it's happened before, to tell you the truth. I guess anything is possible.''

She said she did not know how long the work crew had been on the site.

She said the crash caused an estimated $896,000 damage to Amtrak's train, $202,000 to Burlington Northern's equipment and $13,000 to the track. Crews spent much of the day Tuesday clearing wreckage from the track near Russell, a south-central Iowa town of 590 residents.

Of 248 passengers and 22 crew members listed on Amtrak's manifest, 126 were injured, only one seriously. Amtrak crewman Richard Dillon, 38, of Chicago, remained in serious condition with multiple injuries at Mercy Hospital Medical Center in Des Moines.

Also at Mercy, in satisfactory condition, was Mayme James, 59, a dining car waitress from Chicago.

She said she and two other crew members were in the diner preparing to set up for lunch when the train derailed.

''It just happened so fast,'' she said. ''All I know was I was sitting and I got hit in the stomach with a table.''

Campbell, in a telephone interview from his room at the Lucas County Memorial Hospital in Chariton, said he was dispatched to accompany the Zephyr because the Amtrak crew was unfamiliar with the territory.

Campbell said that to avoid the work crew, the eastbound train had crossed over to the mainline westbound track at Chariton, about six miles west of Russell.

As soon as the open switch was noticed, Amtrak engineer Jim Salmons of Bloomington, Ill., hit the emergency brake, he said. The three crew members, including a fireman, prepared for impact.

''I'd say we were going between 50 and 60 (mph)'' when the crane was hit, said Campbell, 61, of Galesburg, Ill. ''The crane was only a train-length away. We didn't have a chance.''

The emergency brake didn't have time to reduce speed by very much before impact, he said.

''Our engine was thrown to the left. We had to break a window with a fire extinguisher to get out. There was a man and a couple of women there who helped us out. Thank God for that,'' he said.