An Amtrak train carrying 84 passengers crashed into a truck at a rural rail crossing in northern California Wednesday, sending the engineer to a hospital and knocking the train out of commission, officials said.

The northbound train hit the trailer of a semi that was "obstructing" railroad tracks around 11 a.m. in rural Tulare County, said Amtrak spokesman Marc Magliari.

Magliari did not know what caused the collision, saying it was still under investigation. But Tulare County Fire Capt. Larry Stucker told Fresno's KMPH-TV that the big rig had stopped at a stop sign, and that it appeared the truck driver thought the trailer was clear of the tracks.

The engineer was taken to a hospital for possible back injuries.

Amtrak said no one else was hurt, but Stucker said two people were treated at the scene for undisclosed injuries.

The train had been traveling 60 mph and went another 900 feet before it could stop, The Fresno Bee reported.

The big rig was carrying 45,000 pounds of non-toxic potassium chloride, Stucker said. The entire granular cargo was scattered over the road, and its cleanup prevented the road from reopening until after 2 p.m.

Magliari said just the train engine and truck were damaged. The passengers were transported by bus to another train.

The collision took place near Alpaugh, a tiny community 70 miles south of Fresno, or about 50 miles north of Bakersfield.