I have been trying to get into the M3S since launch day, with no luck. There are 2 versions of the issue.
One set of trucks allows me to open the doors (not remove them), the other allows me to remove the doors (bot not open them).
Neither versions allow me to enter the truck (there is no option), nor to view the engine parts (I have even tried crawling under, and climbing on the roof, to no avail)
This is across the whole spectrum of modded servers with no seeming common link, expansion mod too. Obviously I am not going to try Vanilla, those days are long gone.