Cost of train/plane travel

I want to go to Geneva, Switzerland, next month for a couple of days to visit some friends. I don't travel often, and I want to enjoy the journey, so the obvious choice would be to go by train.

I'd get on the Eurostar in London and get off in Paris. Then a quick trip on the Metro from Gare du Nord to Gare de Lyon, where another train would whisk me directly to Geneva. I wouldn't have to worry about getting to and from airports, it would be a nice, leisurely journey, and I'd see the countries I'm travelling through. I'd even be able to use my mobile phone, take some photos, walk around comfortably....

I'm on a budget, though! I can get a return flight to Geneva for about £50. Return. Want to know how much the train would cost? Closer to £200. I could go there and back 4 times on the plane for that money! If money were no object, of course, I would take the train, but money is an object! I need my pennies!

This seems completely backwards to me. A plane takes only a couple of hours (so provides a "better" service in that respect), needs a huge amount more fuel to do its job as it has to lift such a heavy load to such a great height, at a great speed, for a long time. The train doesn't need anywhere near as much fuel, takes 3 times longer, and can carry more people. Why does it cost so much more to go by train?

I don't even want to go by train because it would save energy/CO2 emissions, but because it would be more memorable and enjoyable. Why are train prices so expensive?

Thank you, train companies (or damn you, plane companies) who are forcing my economic hand into making an unwise decision. I suppose the question is not "why is train travel so expensive?", really, but "why is air travel so cheap?" Considering what a plane has to do to get to its destination, I simply can't understand how it can be that cheap, at least compared with the train ticket (which, I should stress, is a cheaper "youth" ticket: it's not even full price!)

Rant over: I'm off back to work.