"A Safeguards Transporter (SGT) is a truck developed for and used by the National Nuclear Security Administration (NNSA), a semi-autonomous affiliate agency of the United States Department of Energy, for the ground transport of nuclear weapons in the contiguous United States. SGTs' trailers are known as Safe Secure Trailers (SST)."
If a SGT comes under attack, unspecified security features in the vehicles give them, according to the NNSA, the capability to "surprise and delay even the most aggressive adversary". The full range of defensive components in SGTs is unknown, but according to some media reports the vehicles are equipped with autonomous weapons systems and other "high-tech surprises" that allow them to independently engage and repel attackers even if all human crew have been killed or disabled. The NNSA has also stated that access to nuclear weapons held within a SST is not possible, even for crew members, due to unspecified security features that prevent the doors from being opened except in "an approved security area".[2][5][6]
https://en.wikipedia.org/wiki/Safeguards_Transporter