The crossing gate was down, lights were flashing and bells were ringing when a big rig crashed into a passing Amtrak passenger train on Monday afternoon, an Amtrak official said.

Investigators probing the cause of the crash south of the farming community of Hanford plan to look at the condition of the big rig's driver - identified on Tuesday (local time) as Macario Medina, 32, of McFarland - and of the truck.

Amtrak spokeswoman Vernae Graham said 39 people were injured in the crash.

Authorities have described the injuries as mostly bumps and bruises although Graham said at least one person suffered a broken leg.

"The whole crew was shaken up, obviously, so what we do is give them immediate relief from duty if they need it and have counsellors check in with them within a 24-hour period," she said.

Investigators will first try to determine Medina's state and then look at the vehicle, said California Highway Patrol spokesman Jerry Pierce. Medina went through the warning arms and hit the train before his truck overturned, according to the CHP.

Pierce said on Tuesday morning that he had not yet interviewed Medina, who suffered moderate injuries.

The impact from the truck pushed two of the train's four cars and its locomotive off the tracks.

The train travelled about 600 feet (180 meters) after the collision before hitting a switchback [a set of points] and derailing, the CHP has said.

Officials have not determined how fast the train or the truck were going, but the average speed for Amtrak through the area is 70 mph to 80 mph (112kmh to 128kmh), while the speed limit on the roadway where the truck was travelling is 55 mph (88kmh), according to the CHP.

After the crash, metal pieces from the truck could be seen inside the train, which was covered by cotton seeds. Several pieces of luggage were also scattered around the area.

The train was on its way from Oakland to Bakersfield. It was being pushed by the locomotive and was struck between the locomotive and the last car, Graham said.

The track, meanwhile, reopened on Tuesday morning after crews replaced hundreds of feet of damaged track and some signal equipment, BNSF Railway spokeswoman Lena Kent said. BNSF owns the line.

- AP