First off, pat yourself on the back for taking the train instead of a car or plane. While they're not 100% pollutant free, they're certainly on the right track!
Next add in your start station and destination, including any stops along the way, to estimate the amount of fuel required.
If this is a frequent journey, like a commute, you can add that into the calculator to give you your footprint over a certain time, rather than just one day.
Next we factor in the train type to adjust the fuel usage estimate, based on how big the train is, how many people share the emissions and what it runs on.
We're here on the other end of the internet, happy to answer questions about calculating carbon footprints or our growing movement.