An Amtrak (search) train derailed after running into a pickup-sized boulder, authorities said. Six people were injured, none seriously.

The California Zephyr (search) struck the 12-by-12-foot boulder late Tuesday just west of Grand Junction, Mesa County sheriff's spokeswoman Susan McBurney said.

The train was en route from Chicago to suburban Oakland, Calif., with more than 100 people on board.

Three engines and three to five cars went off the tracks in Ruby Canyon (search), McBurney said. The site is near the Utah state line.

An Amtrak spokesman, Clifford Black, said the accident injured four crew and two passengers "in the category of bumps and bruises." They were treated at a Grand Junction hospital and released, McBurney said.

The other people were being taken to Salt Lake City by bus, Black said.

Western Colorado has been hit by a series of storms packing heavy snow at higher elevations and rain in the valleys, though when the rock ended up on the tracks was unclear.

"I know the conditions last night were very wet and rainy," McBurney said.

The tracks are owned by Union Pacific Railroad (search), whose spokesman, John Bromley, said finding out when the tracks were last checked for obstructions would be part of the investigation.

"We're all very thankful that there were no significant injuries," McBurney said. "Certainly it was a scary thing for the passengers."