I think we should get some sort of Eurostar Route as a way to expand the SEHS route and I think it would be really good in TSW
London St Pancras to Paris Gare Du Nord with the 373 Eurostar and the 374 Eurostar. However, due to licensing it will be hard, but maybe in the future .
Do you really want to have a 50km long tunnel in the game? The entire route would be around 400km long. So you would need to buy four routes and link them together (if we account for the lengths of all other routes that have been released)
Besides the obvious licensing issues, I worry about the gameplay too. The channel tunnel just wouldn't be fun. Maybe a eurostar route on the continent (to Paris or the BeNeLux). Then the route could see not just eurostar, but TGV or Thalys services aswell.
