Doesn't sound like this system is safety critical. You should be more worried if some hacker can change train signs from stop to go. If you ever ride on a train and see steel boxes by the side of the track, those are control systems and they run up and down the line. They might be locked, or possibly alarmed but that's about the extent of their protection. A simple attack would be to just take an axe to one, or set fire to it. A more sophisticated attack could snoop on the profinet traffic and do something evil.