Looking around the ET archive, relevant stuff:

• A back-of-the-envelope calculation of mine about the power use of a high-speed train and an A340.
  
• A later follow-up, with comparable numbers calculated for litres of fuel per passenger-kilometre, assuming electricity for trains from petrol-burning power plants.
  
• Afew's Air Fare Blair diary.
  
• A discussion on cars vs. trains with focus on consumption.

I also looked up a source on CO2 emissions themselves, it's from German railways 2004 report, all numbers are gram CO2 per ton-kilometre:

• Freight transport: train 29, river barges 35, road (lorries >3.5t) 96, airplanes 665
  
• Local passenger: buses 77, trains 98, average private car 148
  
• long-distance passenger (includes high-speed ICE): buses 33, trains 52, average car 147, [domestic] planes 183

Note that train CO2 emissions can be reduced to near zero by changing the structure of power generation...

_{*Lunatic*, n.
One whose delusions are out of fashion.}