The paper introduces the alternate double single track (ADST) as an alternative to double track lines and discusses its main advantages and shortcomings. An optimization program for the design of such lines are presented, including a detailed analysis of the selected objective function and corresponding constraints. In particular, some interesting formulas for deriving the upper bound of the capacity of these lines are derived to illustrate how the efficiency of the line can range from the single to the double track extremes. Some possible international applications of this alternative are identified in which the ADST could be advantageous. Finally, some real examples of applications, which include the cases of Spain, Chile and Ireland, are used to illustrate the proposed methods.