I could not have articulated this better than Jon Worth in his Lyon-Paris vs. Manchester-London post.
This compares the two segments and how they are served by air and rail: guess who wins?
Just another statistic: on the London-Paris segment, the Eurostar has a 70% share on passengers (and they're carbon neutral). How many airplanes does it displace?
I guess this further proves the point that the Heathrow expansion case is flawed.
Read also my other related posts: richmondtransits.blog/railways