Hearing on Wednesday to focus on how Fed would replace interchange cap and network rules.
The credit union is making the addition through a deal with Dolphin Debit, the same ATM provider that is driving CU24's EasyATM program.
Judge raised the possibility of ordering card issuers to pay merchants while he gave Fed another week to explain how it would amend cap rule.
Angry Judge Richard Leon tells Fed its attorneys can "came come off vacation" and raises specter of damages from overcharges.
Whether or not the Federal Reserve can or will appeal the July 31 court decision that overturned two-thirds of its debit interchange regulation hinges on several unresolved legal questions.
ATMIA says decision last week cancels out a year of work on enabling U.S. payment network use of EMV standards.
Judge didn't hint outcome in oral arguments. Unresolved questions - including legal standing - about ability to appeal.
Justice Department, Federal Reserve also can appeal the Wednesday ruling that threw out the 21-cent cap set by Durbin Amendment.
Two instead of four unaffiliated payment networks could be required as result of federal judge's ruling.
Trades also indicate possible appeal after federal court throws out Fed's 21-cent cap.