Eurostar said that the doomsday scenario was “misleading.”

“We have worked closely with governments on both sides of the Channel, including the Department for Transport, to ensure that we continue operating in a deal or no-deal scenario,” Rosie Jones, a spokeswoman, said in an email.

“Eurostar has been working extensively with our station partners, governments and control authorities on both sides of the Channel to ensure that robust plans are in place to protect services and to manage customer flows effectively,” she added.

For now, the Eurostar shoots back and forth every day, beneath the Channel, catapulting its traveling tribe between the European capitals. Private and professional lives have been formed and played out through this linkup, and careers will continue to be built around it in a post-Brexit world.

Mr. Rahman’s entire adult life is a case in point. A Briton by birth, he quickly became drawn to European Union policymaking and specialized in the subject at university.

He joined the British Civil Service as an entry-level professional in a special program that fast-tracks promising graduates with expertise in the European Union.

Having spent three years in Brussels on secondment from the British government, Mr. Rahman jumped to the private sector, where he is a prominent a nalyst , heading up the Europe practice at Eurasia Group, a political-risk consulting firm.

He says the seeds of Brexit were sown from the start of Britain’s relationship with Europe.

“Britain always sought to extract economic benefits of being a member of the club, without any of the political consequences of being one,” Mr. Rahman said. “It sought to retain sovereignty to the greatest extent possible, it never signed up to political integration.”