On the train from London to Brussels just as the high-speed train dived under the English Channel, Mujtaba Rahman frantically typed an email on his smartphone and hit send.
A short ride later, through a dark void that briefly blocked both his ears and his cellphone reception, he was hurtling across the rolling fields of France. “On the way in and out of the tunnel is a really good time to do some thinking,” Mr. Rahman said.
What he thinks a lot about these days is Brexit. Mr. Rahman once worked for the British government, helping tie its fortunes ever more closely to the European Union. Today, he consults private clients on the risks of Britain’s withdrawal from the bloc.
He is part of a tribe of lawyers, economists, journalists and trade experts who helped turn the high-speed train, known as the Eurostar, into the embodiment of the idea of the near-borderless world that the European Union stood for.
Ironically, the Eurostar, which turned 25 last week, has become the primary vehicle they use for going back and forth to undo the relationship they once helped build.
For those living in London, the Eurostar has made weekends in Paris (or Brussels, or now even Amsterdam) — or vice versa — simple, fast and relatively affordable. It enabled a professional class to bounce back and forth under the English Channel that once served as a moat — protective or isolating, depending on the traveler’s perspective.
These days, regular passengers include the British negotiators working on pulling their country out of the European Union, who have often boarded the train twice a week to head to Brussels for talks about the divorce.
And then there are the others, like Mr. Rahman, whose professional lives revolve around the links that are being severed.
If Brexit eventually happens, as seems likely, the ease of travel they have depended on, contingent on quick immigration checks for European Union passport holders, seems certain to change.
The fate of the train itself may hang in the balance. For now, the executives who run it watch and wait.
Tickets are still selling fast. In August, a record total of one million passengers rode the Eurostar between London and its destinations of Paris, Brussels and Amsterdam. The company reported booming profits in 2017 and 2018, and has strengthened its marketing, bolstered by American demand and a growing preference for green travel.
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.