[mbreese]

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.