The crash Thursday killed four people and injured 16 others, The Wall Street Journal reported.
The flatbed truck was one of two carrying veterans in Midland's annual salute to wounded military and was driven by a young soldier in uniform who survived the crash.
The 7,200-foot Union Pacific train was traveling at more than 60 mph, below the limit for that stretch of track, the National Transportation Safety Board told reporters Saturday.
The engineer sounded his horn and crossing warnings and barriers functioned normally, the investigators said.
The NTSB is interviewing witnesses and reviewing video from the train and a police car that was nearby, the report said.
Early speculation among witnesses is that the truck driver was distracted by the parade noise and activities, but that wasn't certain, the Journal said.