This is fun because a firm - not
bristows - had a big scandal with their associates running their mouths about trainee applications on the train.
As a rule if you become privy to any relevant confidential info on the train - internal or external - you report that either to your direct report or to anyone within the firm to whom that function is designated.
In the hypo you gave, the point isn't to tattle - it's that a potential breach of oppo's privilege has arisen and you could have a mistrial if it later arises that your side was even potentially unduly privy to oppo planning. You're not just protecting like justice and principle by disclosing, you're protecting the credibility of your own case.