Or how about there are two routes - ECML and WCML - which happen to have termini 10 minutes walk from each other, but are otherwise completely unrelated? I'm sure there are plenty of other examples which could be presented like that, but where they are actually completely unrelated, and for perfectly good reasons.
Not least, as an ECML user, I would be pretty hacked off if the King's Cross S&T team were called to a WCML incident, and as a result unable to deal with something 10 minutes later on the ECML.
I work outside the industry, but see how workforce estimates for maintenance teams are prepared. They're based on understanding an area of coverage, and having enough people to cover when something kicks off. That's based on averages, with allowance for the reality that when things go wrong, they really go wrong.