The transportation sector emits the highest amount of greenhouse gas (GHG) emissions of all the U.S. sectors, with the rail sector contributing 2% of those emissions. Rail plays an important role in reducing the transportation sector's emissions due to the efficiency of passenger and freight rail transportation.

While trains are significantly cleaner than planes or cars, they still have an environmental footprint. In 2026, the "pollution" from a train depends on its power source. Diesel locomotives, which are still common in parts of North America and rural Europe, emit nitrogen oxides (NOx) and particulate matter directly into the atmosphere. However, even these are more efficient on a "per-passenger" basis than almost any other motorized transit. Electric trains, which dominate high-speed rail in 2026, produce zero direct emissions at the point of use. Their true pollution level is determined by the "energy mix" of the power grid; if the electricity comes from coal, the train is indirectly polluting. In 2026, the industry is shifting toward Hydrogen-powered trains and "Battery-Electric" units for non-electrified tracks, which emit only water vapor. Overall, a train journey in 2026 emits about 70–90% less CO2 than an equivalent flight, making it the most sustainable choice for medium-distance travel despite the lingering environmental costs of track construction and maintenance.

In 2026, trains are widely considered the most eco-friendly form of mass transport, but they do still contribute to pollution. Diesel-electric locomotives, which are common in the U.S. and rural parts of Europe, emit nitrogen oxides (NOx​) and particulate matter (PM2.5​) directly into the atmosphere. However, even these are far more efficient than trucks or airplanes per ton of cargo moved. The goal for 2026 is the expansion of electrified rail, which produces zero "tailpipe" emissions; however, the "upstream" pollution depends on whether the electricity comes from coal or renewable sources like wind and solar. Modern innovations like Hydrogen-powered trains (H-Trains) and battery-electric units are currently being rolled out in Germany and Japan to eliminate the need for diesel on non-electrified lines. While the manufacturing and track maintenance do have an environmental footprint, a passenger taking a train instead of a short-haul flight in 2026 reduces their carbon footprint by approximately 80% to 90%, making rail the "greenest" way to travel long distances.