(CNN) Investigators looking into Wednesday's deadly crash involving a train carrying GOP members of Congress are focusing on the actions of the driver of a truck the train struck, a source with knowledge of the investigation tells CNN.

Eyewitnesses have told National Transportation Safety Board investigators the truck driver -- who is alive but in serious condition, according to authorities -- was seen trying to snake his way through the crossing gates, despite signals that included lights warning of the oncoming train, two sources with knowledge of the investigation told CNN.

The Amtrak train, which was chartered by GOP lawmakers to take them from Washington, DC, to West Virginia for their retreat, was traveling at approximately 60 mph at the time of the accident. The tracks where the accident happened are not often active passenger rails and are usually only used for freight trains, which travel much more slowly, usually around 30 mph.

Locals who live near these tracks learn the routine and they plan accordingly, the source said. This Amtrak train was going roughly twice as fast as trains that usually travel on these tracks, so the truck driver may have miscalculated how much time he had if he mistook the Amtrak for the usual slower freight train.

Although it's unusual to have passenger service on the rail line, the tracks are rated for 60 mph, so the fact that it was traveling at this speed is not thought to be an issue, one of the sources said.

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