As of the moment, there is only one way to travel between the United Kingdom and France, Belgium, Germany, Switzerland, and the Netherlands by rail, and that is through using the Eurostar service that passes through the Channel Tunnel. The daily return train service travels between London, Paris, Brussels, Antwerp, Lille, Rotterdam, and Amsterdam, with occasional other routes stretching to Cologne, Dusseldorf, Zurich, and Geneva. While it's a handy option, it does have a monopolised grip on the rail routes, even if this won't always be the case.
It has been confirmed Virgin Trains will soon start operating services between London, Paris, Brussels, and Amsterdam, all as part of its own Eurostar rival. This service has recently been given the greenlight, and according to The Telegraph, there are plans to kick off service with as many as 20 daily return trips, all starting from October 2030.
The reason as to why the service won't open for another four years is because Virgin Trains now needs to secure approval from British and European authorities to operate its trains on the planned routes, with ideas circulating by Virgin founder Richard Branson to even re-open the train services between London and Disneyland Paris, and potentially even exploring a line that accesses Paris' Charles de Gaulle airport too.
Expect more news in the future as Virgin continues its efforts of taking on Eurostar in trans-European rail transport.