A very interesting study found on the internet, evaluating aircraft and High Speed Train (HST) operating costs by Moshe Givoni leads to the conclusion that HST might be a real incentive for an airline to shift its PAX from an A/C to the train only at peak hours and with respect to environmental costs.
The study focusses on the CDG-Heathrow route and takes only operating costs (not the financial and infrastructure sides) into consideration, and this with a lot of assumptions. But it's a very interesting paper, suggesting that more could be done to study rail alternative on short haul.
Sorry, I lost the link, so please go Google with the author's name.