[Someone1234]

To be honest when it comes to critical infrastructure, you cannot be conservative enough.

Which is to say: These networks should be closed-loop, and air gapped from the internet. The best security is simply not letting an attacker connect to the equipment at all.

Same way cars are meant to work. One network for the vehicle control systems, another network for the fun stuff (in car entertainment, OnStar, etc). With well defined interconnections between them (that assumes the "fun stuff" side can misbehave).

If there is any interconnection between the critical infrastructure network and the internet, then it needs to be very well regulated, down to the packet lengths.