Apparently the gun is removed when the vehicle is in transit.
I was surprised at first, but it seems to be "common practice" in order to avoid damages during transportation.
Apparently an incident occurred in Poland back in 2017 to that effect:
"None of the American institutions has so far signed as the formal owner of the armored vehicles. It is not known whether it is a division stationed in Poland, the command that sent the wagons to Poland, or maybe the Pentagon. The liaison officer stated that he was not authorized to do anything, and without identifying the owner, there was no victim and he could not be questioned. Secondly - you can't determine the value of the Bradleys, and without that, specific losses. Hence the letter from the investigators to the US Embassy, because as our journalist heard, without the cooperation of the Americans, the investigation is in danger of being discontinued."
So, one might wonder why taking such precautions with a vehicle that is supposed to have a rough time on the battle field, in conditions much worse than a trip on a train cart. The reason seems to be "paperwork in case of an accident".