Air quality Air quality data 1 Beijing [China] 1 China 1 Difference-in-differences 1 Distance-based 1 Health benefits 1 Major cities 1 Railroads 1 Standard deviation Subway 1 Subway network 1 Subway stations 1 Subway systems 1 Subways 1 Traffic congestion 1 air quality 1 atmospheric pollution 1 empirical analysis 1 morbidity 1 public health 1 quantitative analysis 1 traffic congestion 1