I was part of a team hired to create training videos on the banking system "updates" after the OKC bombing. This was the first time I had a look under the hood of a redundant system with Primary, Secondary, Tertiary, and further systems.
Primary and Secondary were running 24/7 with Secondary being in exact sync with Primary. If Primary or Secondary went down, Tertiary could be brought up in sync within 24 hours. Primary and Secondary were geographically separated (east/west coast). Tertiary was in the mid-west. After that, the additional facilities would take days to bring up, and then a bit of time to sync.
The specific bank is no longer alive as it was consumed by one of the "too big to fail" institutions.