Those ECROs who come from the outside (not unusual, at least in my neck of the woods) are generally from the utilities sector these days. However if you've got this far you're obviously what they're looking for for interview regardless of your background
Regarding the interview, I'm not sure exactly what it entails but research the Network Rail non - technical skills; control under pressure, working with other people; communication skills that kind of thing. I don't know much about the situation at Rugby because A - I'm not an ECRO and B - I'm in a DC area and Rugby is AC. I think Rugby ECO is on SCADA now but I'm not sure. However you have to deal with maintenance staff; engineering possession staff; depot staff; Control and signallers on a daily basis (usually all at the same time after sitting idle for five hours, as is the nature of the beast unfortunately) so the most important thing to stress in the interview is that you can keep your head whilst all around you lose theirs; perhaps more so than any other railway role. Hope that helps