Put decals or other signage on the interior and exterior of train cars so that center doors are exit-only and left and right doors are enter-only to help passengers board more quickly.
One of the most frustrating things about Metro is the pushing and shoving that sometimes takes place on crowded platforms to board the train. People crowd around doors to board quickly but are instructed to let others off the train before they board. This delay in boarding sometimes prevents people from being able to board in a timely manner even when there is room on the train for them. Furthermore, it is difficult to get off a train quickly when there is a crowd of people in the way anxious to board. I propose that Metro designate center doors as exit-only and the two other doors on the train car as enter-only (except in the case of an emergency in which all doors may be used to exit.) Designating the center door as exit-only will actually encourage people to move toward the center of the car and make it easier for people to board through the left and right doors.