The train design problem (also called "block-to-train assignment problem") is a difficult combinatorial optimization problem encountered daily in the freight railroad industry. In 2011, the Railway Applications Society (RAS) of the professional society INFORMS had set up a competition problem of this based on a simplified real-life instance, which is the basis for this chapter. In this chapter we discuss this problem, and develop an algorithm for it based on the minimum cost spanning tree approach, and the solution of the RAS 2011 contest problem using it.