> There's also this weird thing where, while the train is sitting there waiting to return the opposite way back to Brooklyn, they close all the subway doors exc

> There's also this weird thing where, while the train is sitting there waiting to return the opposite way back to Brooklyn, they close all the subway doors except one in each carriage. It doesn't prevent people from getting on. My understanding is that it's to keep the subway cars relatively climate-controlled. Especially in the summer, it's better not to let all the cool air from the AC leak out the open doors.