A banner across the truck's front bumper read, "Heroes on Board."
The parade was to end at a "Hunt for Heroes" banquet honoring the veterans. The wounded service members were then going to be treated to a deer-hunting trip this weekend. The events have been canceled.
Lange said Union Pacific is offering help to the community and victims' families, as well as peer-to-peer counseling for the train crew, who did not sustain any injuries.
"There is going to be a very thorough investigation," Lange said. "It's obviously a very tragic incident."
The National Transportation Safety Board also is investigating, NTSB spokesman Peter Knudson said.
Midland is about 320 miles west of Dallas.
Associated Press writer James Beltran contributed to this report from Dallas.