Developing high-speed rail—transportation that's at least twice as fast as cars and can be more convenient than flights—has the potential to reduce carbon emissions in the U.S. by gigatons. There are many examples of impressive high-speed rail overseas, from Japan's Shinkansen to France's recently unveiled TGV Inoui—and while the U.S. is currently behind, experts think that one high-speed rail project currently in the works could be the tipping point for buildout across the country.