I have IC and I worked in Boston for 2 years, commuting in by train. Boston is better than NYC or Chicago in regards to bathrooms without a door code in my experience,but if there is a door code and you don’t look threatening, the employees will usually give it you quite readily.

Everyone here is giving really good advice, I’ve peed in all the nicest hotels in Boston 😂The worst part for me was always the lack of rest rooms in the train stations and the state of them if they do exist.

In short, I don’t think you have much to worry about. Just keep in mind that if you have a long journey on the T, make sure to go before you get on! The commuter rail does have restrooms in their double decker cars and it’s pretty easy to use them as long as it’s not peak commuter hours.