I’m reminded of when an old AT&T building went on sale as a house, and one of its selling points was that you could get power from two different power companies if you wanted. This highlighted to me the level of redundancy required to take such things seriously. It probably cost the company a lot to hook up the wires, and I doubt the second power company paid anything for the hookup. Big Bell did it there, and I’m sure they did it everywhere else too.

Edit: I bet it had diesel generators when it was in service with AT&T to boot.