Our results suggest that HSR may on average have some but limited pollution-increasing effects, as a result of its economic promotion effect, but also reduce pollution emissions in heavily polluting industries and central areas due to its industrial and spatial restructuring effects.

High-speed rail (HSR) is widely considered the cleanest form of long-distance mass transit, but it is not entirely pollution-free. Most HSR systems, like the Eurostar or Japan's Shinkansen, run on electricity; therefore, their operational emissions depend entirely on the "energy mix" of the power grid. If the electricity is sourced from wind, solar, or nuclear power, the train's carbon footprint per passenger is nearly zero. On average, HSR produces about 23% to 30% fewer emissions than flying the same distance. However, significant pollution occurs during the construction phase, which involves massive amounts of steel, concrete, and heavy machinery (like tunnel boring machines). A 2026 study suggests that an HSR line must reach high ridership (often 10 million trips annually) and operate for several decades to "offset" the initial carbon cost of building the tracks and tunnels. Additionally, HSR can cause noise pollution and "habitat fragmentation" for local wildlife. Despite these factors, HSR remains the most sustainable alternative to short-haul flights and car travel in the global fight against climate change.