English: Third rail at the South Station stop on the MBTA Red Line. While the third rail shown here may look old and grungy, it is actually advanced mass transit technology. Third rail is subject to heavy wear and must be made out a strong material, typically steel. Unfortunately, steel is a relatively poor electrical conductor. A new method for reducing the resulting resistive losses is to attach strips of aluminum (which is a better conductor of electricity) to the steel third rail. Since aluminum has a different coefficient of thermal expansion from steel, the strips must be applied on both sides and riveted at frequent intervals. The third rail pictured here uses this method.