James Strickland has created a great webpage detailing energy consumption of various modes of transportation, highly recommended.

Leaving the energy equation aside, here are some more reasons taking a train instead of flying makes sense.

Doubling effect of my support
When I take the train instead of flying my financial support is shifted causing a doubling effect. For each dollar not spent on flying, one is spent on rail, thus achieving a $2 impact for each $1 out of my pocket.

Demand increase - decrease and public investment
By creating demand for rail, while decreasing demand for air, public investment in infrastructure also shifts. Greater investment into rail will result in more reliable and higher quality service while simultaneously reducing the investment into infrastructure for air travel, thus reducing the quality of service for air travel and making rail more attractive.

Climate Change (Global warming) & Pollution
Climate change induced by global warming is caused by emissions from burning fossil fuels. Jets burn an unregulated crude form similar to kerosene in quantities so vast they are measured by tons rather than gallons. The emissions are deposited freely upon the neighborhoods below until sufficient altitude is reached to suspend them in the upper atmosphere, where they combine and double the greenhouse effect that is causing global warming. Trains use half as much energy and leave no contrails.

Political Support
Follow the money. Who benefits from increased consumption of fossil fuels?

10 MORE REASONS FROM A UK EARTH FIRST GROUP