Handily the Guardian Newspaper has published a comparison of CO2 emissions between plane and train (and car). See https://www.theguardian.com/uk-news/2023/jun/30/carbon-emissions-from-uk-rail-travel-lower-than-previously-thought?CMP=Share_iOSApp_Other

The example the Guardian cited was the journey from London to Edinburgh, which is one part of the journey that I completed. 12.5kg for the train and 131kg for the plane, more than a factor of 10 difference. All the trains I travelled on we’re pretty well full, so maximum savings achieved.

From the article “The first result from RDG’s new carbon calculator confirms this figure is actually 12.5kg/CO₂e – approximately half the previous estimation, and 10 times less carbon than car travel or 13 times less than the equivalent flight.” referring to trains. I am actually surprised how much better trains are than flying and cars.