A driver working for John Davis Trucking of Battle Mountain smashed through crossing gates and into two double-decker cars of an Amtrak train Friday about 70 miles east of Reno.
The train was headed from Chicago to Emeryville late Friday morning when the accident happened.
The drivers were part of a three-truck convoy that saw the gates come down and the warning lights go off as the California Zephyr approached, National Transportation Safety Board member Earl Weener said Saturday.
They stopped, but the driver of the big rig in the lead did not, he said.
Six people were confirmed dead including the truck driver and the train conductor, while 28 remain unaccounted for.
Officials say some passengers may have gotten off the train before the crash or walked away from the scene without informing authorities.
Federal records reviews by The Associated Press show the Nevada Department of Public Safety cited the company for two crashes in the last two years, including one in February 2010 that injured a person in Washoe County.
In a January inspection, authorities found tires on one company tractor-trailer so bald they deemed the rig an imminent hazard to public safety.
The Associated Press contributed to this report.