The cost of infrastructure is extremely hard to quantify. Roads aren't free either, and parking in cities makes them less dense and hence increases the distances people need to travel.

Where does the CO2 cost of the infrastructure come in? Is that just in construction or do rails somehow emit CO2? Or are you just saying that rails take up space and that's an environmental impact too?

I feel like I'm missing something.

I’m guessing it is in the production and transportation of materials to the site. Subway stations require a lot of material to build, so you’d have the CO2 cost of the concrete, rebar, etc... rail lines don’t have much material, but producing steel does take a lot of energy. And if the line is between large metro areas, the rails would need to be transported to (potentially) remote areas.

But I’m guessing that in terms of CO2 impact, these are fixed costs. So, it would seem reasonable to me to assume the operating CO2 costs would dramatically outpace the fixed initial costs, hence why it would be ignored.

Or potentially, when comparing two systems, the initial building/infrastructure CO2 costs may be assumed to be roughly equal, so are factored out.

The German newspaper article names construction and operation of railway stations, and the construction of the infrastructure itself.

My main point is: there seems to be a non-neligible impact, and CO2 calculators don't help me grasp that.

Is the German newspaper also considering the infrastructural costs of road construction, expansion and maintenance (particularly maintenance costs incurred by shipping freight on those roads instead of rails)?

Forget newspapers. Here's an IRU study about combined rail/road freight transport accounting for costs and emissions.

"There is no such thing as a truly environment friendly means of transport. Combined transport is not inherently superior to pure road transport in terms of environmental impact, as measured by energy consumptionand CO2 emissions."

https://www.iru.org/apps/cms-filesystem-action?file=PPP/en_C...

This “study” is literally from an organisation dedicated to keeping freight transport on roads, it’s hardly unbiased, and is actually of very poor quality - I’d expect to see, for example, an explanation of how the routes involved were chosen for the study. Additionally, if the study said anything other than “road freight is good”, it just wouldn’t have been published by this organisation!