I went to London recently by train (and returned). On my ticket receipt, Last Great Western noted that they had contributed 29.54 kg to the atmosphere on my behalf to get me there. Had I gone by car, I'd have contributed 70.64 kg. So well done me.
But I was struck by how relatively small the difference was: 30 v. 70.
So I checked their carbon calculator page. This says: "We have estimated the carbon footprint of your train journey, and an alternative car journey, by multiplying together three pieces of information:
1. The number of passengers (the default is one)
2. The distance of your journey, per passenger. This data comes from WebTIS, the system we use for online ticket purchasing, which in turn takes distance information from the rail industry’s ‘National Routeing Guide’ data feed. The data describes the total distance of your journey, from station to station. If there are multiple route options for your journey, the distance is specific to the route you have selected. If you select a return journey, the travelling distance is set accordingly. For car travel, we don’t know the specific route you would travel, so to give an indicative comparison we have assumed you would travel the same distance by car as you would by train.
3. The average emissions intensity of rail and car travel for every km travelled, per passenger. We have used overall averages for UK rail and car travel, as published in the UK Government GHG Conversion Factors for Company Reporting, 2016. Car data is published per vehicle; to convert to ‘per passenger’ figures we used the national average car occupancy of 1.6 people, based on the Government’s National Travel Survey: England 2015."
This means (I think), that if I were to travel to London and back on my own in a car, my footprint would be 1.6 x 70kg. That is 112 kg.
This makes the difference 30 v. 112 which I still think is small as you'd only need a car with 4 people in it to better the train figures. Maybe the following is part of the answer:
"For national rail, the government data gives one overall average (it doesn’t distinguish among types of train). We use this data so that we can represent your journey no matter where you travel across the UK. In reality, emissions from diesel and electric trains will be different, and we are in the process of introducing new electric fleets across the GWR network. We are currently doing work to assess the carbon implications per passenger of this transition."
I was on an electric train for ~90% of the journey which means that I was right to be skeptical of the initial figures. So hurry up Late Great Western, your electric fleet has been operating for a while now; it's time for more calculation.