I would like to know if it is possible, without social engineering, to attack, pentest and get into infrastructures that are normally considered to be extremely secure and well protected: all the different types of SCADA, Intranets, enterprise servers and/or mainframes and other systems such as the ones that operate telecommunication lines, railway systems, surveillance systems and devices such as aircraft. (I know all those are very different infrastructures, but you get the idea.)
Moreover, what specific knowledge, of what subjects, is needed to get into that kind of security testing? Apart from the obvious (knowledge of TCP/IP, etc), what kind of programs should one be able to understand and write, and what kind of attacks and techniques should one be aware of?
I know my question is vague, but I would like to have a broad view of things, and the skills needed, before getting into specific subjects. Feel free to elaborate on one of the infrastructures I mentioned, though.