The potential growth of the railway traffic is expected to be substantial worldwide; hence railway companies and agencies are looking for better tools to allocate their capital investments on capacity planning in the best possible way. In this research I develop a Multi-period Investment Selection Model (MISM) by using robust optimization and multi-commodity flow techniques. The MISM is then decomposed by using Bender’s decomposition to efficiently obtain the optimal solution. Based on the estimated future demand, available budget and expansion options, MISM can determine the multi-period optimal investment plan regarding which portions of the network need to be upgraded with what kind of capacity improvements at each defined period in the decision horizon. Using this decision support tool will help railway companies or agencies maximize their return from capacity expansion projects and thus be better able to provide reliable service to their customers, and return on shareholders’ investment.