U.S. Steel is currently one of the largest influences in the steel industry, it makes sense to highlight how Pittsburgh has always been known as "The Steel City." Built heavily around the use of railroads, there's also a rich railroad history. 
U.S. Steel still heavily depends on railroads to deliver iron ore and coke to its plants. This leads to the production of steel coils, slabs, and more. Hence, the flatcar with a steel beam on the towel for Pittsburgh Night presented by U.S. Steel.