The undervalued stock is luring Icahn back to Greenbrier at the same time that railcar demand picks up from customers such as energy companies, according to Stephens Inc. A combination of Greenbrier and American Railcar (ARII), in which Icahn has a controlling stake, would overtake Trinity Industries Inc. as the industry leader with more than a third of the market and allow the merged company to cut overhead expenses, said Susquehanna International Group LLP.