Sorry didn’t see this thread earlier! We added a fix for New York MTA ticket machines on the current account just before Christmas: if the ticket machine requests you to enter a zip code for your Monzo current account debit card, you can input any number to proceed. 
We’ll use this workaround for any US self-service terminals that request a zip code (e.g. ticket machines and automated fuel dispensers). We’re fixing these on a case-by-case basis so if you find a terminal or merchant where this doesn’t work, please let us know via in-app chat! 