To plan and control train traffic constitute very complex tasks. The human being, in the role of train traffic dispatcher, must be able to take fast and correct decisions and effectuate them efficiently. These operators must then also be supported by efficient decision support and control systems. In order to study the involved questions a train traffic simulator (TOPSim) has been developed at Uppsala University in Sweden. With this TOPSim simulator it is possible for a human operator to in real-time control the movements of the simulated trains running on a model of an arbitrary railway line. This paper describes the results of our experiments using the first version of the TOPSim simulator, with the focus on questions concerning the computerisation of the re-planning process. Professional train dispatchers have been participating in the experiments presented here. Also the questions of using a decision support tool based on a traditional train traffic simulation program, in the train dispatching process are discussed. These questions have some interesting implications on the traditional use of simulation in the long term planning process.