I'm not sure anyone knows, and I've never known someone to get audited for intracity public transport. It may exist, but I've never heard of it. The fact that most card providers tell you not to do it at least raises some concern they could cancel your account for incorrect usage (violating terms of service does not require a law to be broken).

Your benefits team will likely provide little or no help (just like your card supplier) since the law is ambiguous. They will repeat the same language the IRS has stated ("parking near your workplace" and "transit expenses").

The problem is exactly what you stated. Back to the original subcategory we are talking about. Commuter expenses. “Use it to pay for commuting expenses (bus/train/carpool)".

Would you consider a trip to the grocery store a "commuting expense"? I certainly wouldn't. Will anyone care? I certainly doubt it.