Using Fedline Command with Axway by chance? E.g. see https://www.frbservices.org/binaries/content/assets/crsocms/... matrix of ways to connect to the Fed.

  we integrate with FED over some... interesting protocols.

Would love to know more!

My understanding is they sftp batches of CSV files at eachother.

this is probably closer to the truth that you might have imagined :) that at that most of the $1T+ ACH transfers that happen daily in the US banking system happen over plaintext files and SFTP (source: I've written one of these files)

So I guess that means handling SWIFT MT / ISO 20022 messages is a big part of the custom stack?