Commuter trains are already somewhat eco-friendly by their nature (you're less likely to need a car, after all), but the San Francisco Bay Area's train system is taking things one step further. BART (Bay Area Rapid Transit) has unveiled a policy that will gradually move it to completely renewable energy. It starts off modestly by limiting CO2 emissions now through 2024, but the plans will be more aggressive after that. At least half of its energy will have to come from renewable sources by 2025, with 90 percent of it from low or zero-carbon sources. All of it will have to be zero-carbon by 2035, while complete reliance on renewable sources would come by 2045.