The National Transportation Safety Board issued its final report on the accident Thursday. The crash killed a passenger in the truck and injured a congressman and others on the train.
The NTSB said the probable cause was the truck driver's decision to enter a railroad crossing with the gates down and his inaction after encountering obstacles on the tracks.
The NTSB said his decisions were "most likely due to his impairment."
The driver was charged locally on involuntary manslaughter and DUI.
A judge threw out blood evidence. A jury acquitted the driver of manslaughter last month.