On July 15, 2014, a dealmaker from asset manager Lazard hopped aboard a Eurostar train from London to Paris and typed furiously on multiple mobile phones. He didn't pay attention to his neighbour sporting a three-day stubble and casual clothes. Big mistake.

A conversation on a train put an interesting wrinkle into a $US15 billion potential deal. Credit:Shutterstock

Dealmaker Vincent Le Stradic continued to work during the 2½-hour trip, unaware that he was providing a window into a soon-to-be-announced $US15 billion ($21.1 billion) takeover bid to his seatmate, a UBS banker. Details came to light at a hearing by France's financial-market oversight authority.

The backdrop for the case is the audacious attempted takeover of T-Mobile US by the French mobile-phone carrier Iliad, founded by Xavier Niel. Le Stradic was and still is one of the French billionaire's closest financial advisers, while the man in the seat next door, Alexandre Zaluski, worked in the investment banking arm of UBS in London, focusing on equity capital markets.

During the journey, Zaluski caught a few glimpses of messages that popped up as Le Stradic juggled between phones. Unable to make sense of the piecemeal information into the deal, which would be announced two weeks later, the UBS banker decided immediately to reach out to a colleague via email. Ultimately, the bid was unsuccessful.