They are just better for it than cars and planes.

Check these articles out:

Eco Travel by Train – Train Chartering

Environmental Benefits of Rail – ARTC

Environmental Benefits – Vermont Rail Action Network

Why is rail travel a better choice for the environment? – Transport for NSW

The Economic and Environmental Benefits of Passenger Rail – All Aboard Minnesota

Benefits of Rail, The Environment – Omnitrax

Greater Anglia report highlights environmental benefits of rail – Rail Business Daily