Recent studies have shown that the dynamic removal of transmission lines from operation (Transmission Switching) can reduce the costs associated with power system operation. Smart Grid technologies introduce flexibility into the transmission network topology and enable state dependent co-optimization of generation and network topology. The optimal transmission topology problem has been posed in previous research on small test systems. However, the problem complexity and large system size makes optimal transmission switching (OTS) intractable on real power systems. We analyze the optimal transmission switching results of prior work on the RTS-96 network and find that most of the economic benefits arise from switching a small number of lines. We decompose the results to determine the marginal savings contribution of each optimally switched line. Our marginal analysis leads to the development of an off-line screening method, based on network sensitivities, for identifying candidate switchable lines. When compared to OTS on the RTS-96 network, our screening tool generates near optimal solutions in a fraction of the time.