And if this headline used Train hero as a name (e.g. Train Hero) rather than a description of a person then we'd be closer to a one-to-one comparison. If they used the train hero's name in the headline in addition to calling him hero, then you'd be spot on.

Even then they wouldn't be scare quotes. It would just be a different style choice. I checked the BBC style guide and this is pretty open to interpretation.

No one at the BBC is making a case that an unarmed man stopping a mass shooter is not heroic. This is people looking for stuff to be outraged about.