Depends; if you use renewable energy, operating it could pretty much be neutral. Given that this is in China, I would expect them to mostly power this with rene

Depends; if you use renewable energy, operating it could pretty much be neutral. Given that this is in China, I would expect them to mostly power this with renewables (wind, hydro, or nuclear). They do still build a lot of coal as well of course. Steel production for the infrastructure could long term also be a lot cleaner but is short term likely to have a substantial CO2 footprint. But that would be similar to other high speed rail options. Probably, over the life time of the installation, it would be quite good.