In the news or various other places, we see often that train travel produces significantly less greenhouse gas than air travel, without much information about how this is calculated. For example here : https://reporterre.net/L-avion-emet-1-500-fois-plus-de-CO2-que-le-train
On the other hand, they hardly ever mention if the infrastructure is taken in account. For train travel, stations, rolling material, rails, ballast, viaducts, tunnels, have to be built and maintained, sometimes replaced regularly. When passengers increase, new tracks and new stations have to be built, sometimes infrastructure extension can become very heavy. This is mostly done with construction vehicle running on diesel, and tons of concrete are used. The higher speed the train line, the more heavy is the infrastructure, in particular larger viaducts, longer tunnels, and rails used more quickly.
Air travel, as much polluting as it is, only needs airports and the planes themselves to be built and maintained. There's no need for infrastructure on the way between airports.
Maybe this is already taken in account in the calculation made "ready for the public" on the news, and that despite this the train is favourable. But I prefer to be always skeptical of the info and try to understand how this is calculated.