MARTA said in a statement that the glitch -- which left the machines unable to accept all forms of payment for several days -- was "extremely unusual."
Officials said the problem was caused by a software upgrade meant to comply with the latest credit and debit card security standards.
The Atlanta Journal-Constitution reports that the system only accepted cash between Aug. 31 and Sept. 4. A partial fix, for credit cards, was made on Sept. 4, in time for Tuesday's rush hour. The debit card fix was announced Sunday afternoon.