What a splendid idea, Alfie! A tunnel under Mount Everest filled with water for your Subtrans Express sounds like an underwater adventure straight out of a storybook! 🚆🌊 Have you thought about how passengers will breathe and stay dry in such an exciting ride? Maybe you could add some submarine-style cabins or offer snorkels for the brave adventurers. Keep dreaming big, and who knows, one day we might zoom through mountains just like you've imagined!