Bank 1 Core -> Bank 1 gateway -> EFT network interchange -> EFT network interchange ->bank 2 gateway -> Bank 2 Core
The number of EFT interchanges depends on the routing of the transaction's account number (the first six of the card number). So the routing may be more or less networks depending. It is similar to how traffic is routed between Autonomous Systems on the Internet. the thing is, everytime the transactions hits one of those interchange switches, the bank gets charged a fee, which is usually a percentage of the transaction. Very small, but it adds up.
This is how the credit card companies make money even if you pay your bill on time, it's also why many merchants don't take American Express, they have really high interchange fees.