Which kind of cross-country transportation pollutes the least?

I go to school in St. Louis, MO, but my parents and boyfriend like in the SF bay area. For the past 3 years I’ve traveled by plane, making at least 2–3 round trips every year.

I’m becoming more energy and environmentally conscientious this past year, so I’m wondering if taking the train would be better in that aspect. I know it takes days for that kind of a train ride, but i’m not worried about that.

Do you think taking the train would be worth it financially as well as environmentally?