a rapid discharge railcar door comprised of a main body composed of a polymer reinforced by an aluminum frame , the railcar door being designed to provide sufficient strength against load - bearing forces and withstand changes in temperature . the main body of the door is made from a polymer using a thermoplastic or thermoset molding process , such as injection molding , casting , resin transfer molding , reaction injection molding , and reinforced reaction injection molding . the door reduces the overall weight of the railway car , thereby allowing more coal to be shipped per car .