NTSB investigators have returned to the scene of an accident between an Amtrak passenger train and a tractor and double trailer combination in rural Nevada on June 24, 2011.

As a result of the accident, the truck driver, a train crew member, and four train passengers were killed.

Investigators were on site conducting tests and gathering data to gain more information about the performance of the vehicle and the circumstances and dynamics of the sequence of events leading up to the accident.

At the NTSB's request, John Davis Trucking, the motor carrier whose vehicle was involved in the crash, supplied a truck and set of trailers for the test that were nearly identical to those involved in the accident. The Nevada Highway Patrol and the Nevada Department of Transportation provided support essential to the testing operation.

In the area of human performance, investigators conducted tests to determine if the driver could have heard the train's horn. The train's locomotive event recorder and audio captured by a microphone mounted external to the locomotive revealed that the horn was sounded four times as the train approached the crossing. Based on information from the recordings, investigators calculated the distances between the truck and the crossing during the four horn blasts.

Investigators timed the test to coincide with a regularly scheduled Amtrak train that travels along the accident segment of the railroad tracks on a daily basis. During the test, train horn sounds were recorded from the inside of the exemplar truck as the Amtrak train approached along with ambient noise of the truck-tractor under a variety of driving conditions.

In the area of vehicle performance, investigators gathered additional data about the braking system and deceleration characteristics of the exemplar truck. Testing equipment was installed within the air-operated commercial vehicle braking system to measure air brake lag time on each of the nine axles of the truck-combination unit. The exemplar truck-tractor combination unit was then skid-tested near the accident site in order to gather information about the speed, stopping distance, and deceleration of the combination unit using multiple methods of data collection.

NTSB investigators are continuing to develop additional factual information and are in the process of analyzing the data derived from the tests performed in Nevada. The accident docket, which includes factual reports and hundreds of pages of documents, is expected to be opened within several months. A Board Meeting, where the determination of the probable cause of the accident will be made, is expected next year.

To read more about the investigation, visit: www.ntsb.gov