Public good is a widely used economics term that refers to a good such that, once provided, the good is 1) non-rivalrous and 2) non-excludable. By this definition transit is not a public good.

Non-rivalrous basically means “your use doesn’t affect my use.” Crowding out can occur very easily in transit so it’s rivalrous.

Non-excludable basically means no one can prevent you from accessing it. We install payment systems at the entrance so we do exclude people.

Textbook examples of public goods include national defense and clean air. Public transit should be well supported and financed, but they’re right it doesn’t meet the definition.