I don't intuitively see why trains would be massively cheaper than a circle nowhere interesting (underground). HS2 is too expensive, sure, but the LHC is built

I don't intuitively see why trains would be massively cheaper than a circle nowhere interesting (underground). HS2 is too expensive, sure, but the LHC is built next to ~a century of research institutions and took a very long time, whereas trains can actually be on somewhat tight schedules in the places where it actually matters and requiring bulldozing a fuckload of expensive land, houses, and history etc.