Daniel Maposa and Siphosenkosi D. Swene
In this paper the results of a locomotive scheduling problem faced by National Railways of Zimbabwe (NRZ) are presented. The study is in part inspired by similar optimization problems in public transports. This paper focuses on the planning version of the locomotive scheduling model (LSM), where there are multiple types of locomotives and a decision is made as to which set of locomotives is to be assigned to each train. The goal is to develop excellent plans along a number of dimensions for NRZ. An integrated model (Model 1) that determines the set of active and deadheaded locomotives for each train, and light travelling locomotives is presented. An important feature of this model is that consist-bustings are explicitly considered. Two other models namely Model 2 and Model 3 are also considered in this paper. A mixed integer programming (MIP) formulation of the problem that contains 20 integer variables and 32 constraints is given (Model 1). Using LINGO 10, a solution technique was developed to solve this problem within 2 minutes of computation time. When the solution was compared with the existing scenario at NRZ, savings of 38 locomotives were realised, which translates into savings of over ten million dollars annually.
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