On the Permaculture in Brittany blog I saw a cool description for sizing of a swale which I read translated by google from the original French. 

New commute

Now that I have moved to Redwood City, my commute is a little more complicated.
I used to be able to walk three blocks to work. Now I have about a mile walk to CalTrain, and then take the train to Mountain View, where I pick up a company shuttle to work. The walk takes me about 25 minutes, or I can cut that in half with my skate board.

This leaves me with a couple questions:

• What is my marginal carbon footprint for this trip? (All the trains and shuttles are going to run if I am on them or not).
  
• How would this compare to an all electric car used for the trip?