Improving　air　quality　across　mainland　China　is　an　urgent　policy　challenge,　while　rapidly　increasing　use　of　vehicles　poses　a　great　menace　to　the　urban　population　and　air　quality.　Public　transportation　has　received　increasing　attention　as　emissions-　friendly　transport　options,　but　whether　the　urban　rail　transit　system　has　a　significant　effect　on　curbing　air　pollution　has　been　unclear,　and　there　is　a　lack　of　explicit　case　studies　in　mainland　China.　Therefore,　the　effects　of　the　opening　of　the　Beijing　Metro　on　air　pollutants　emissions　in　Beijing　were　quantified　through　a　regression　discontinuity　design.　Beijing　has　seen　a　brisk　growth　in　its　rail　transit　infrastructure　and　a　downward　trend　of　air　pollution　index　since　2005.　The　regression　results　show　that　the　operation　of　the　rail　transit　system　was　observed　to　have　a　significant　effect　on　reducing　most　of　the　air　pollutants　concentrations　(PM2.5,　PM10,　SO2,　NO2,　and　CO)　but　had　little　effect　on　ozone　pollution.　The　results　obtained　were　highly　robust　across　a　variety　of　tests.　This　study　contributes　to　empirical　evidence　on　the　air　pollutants　associated　with　the　opening　of　rail　transit　and　traffic-related　pollution　control　policy　making　in　Beijing.