Yes they are, but the transaction networks are distributed over dozens of cities and the databases also run in parity across dozens of cities. The knowledge depo for the infrastructure is largely based in Atlanta/Marrietta and I think it is anyone's guess how vulnerable the system would be to a large scale attack during a disaster recovery implementation due to the loss of a major hub and many of the experts that support it. We used to put in dedicated low voltage circuits with token rings to keep the connection itself virtually unbreachable. The transaction bridge gateway was the only public facing module that was slightly vulnerable, and then only vulnerable to the occasional invalid authorization. If you wanted in the system, you would need to connect at a secured location through the token ring. The data involved with real time processing and security has pushed large portions of the banking and transaction networks onto the public network with fiber shares that rely on virtual security over the previous physical security. If a major hub like Atlanta went dark, it seems plausible that it could be a piece of a much larger financial attack.