This paper describes a realtime digital train simulator which was realized to develop real braking devices. The main originalities of this simulator consists in the solution introduced in order to allow a true simulation of the reality at reasonable price. In particular, in the modelling, we simplified the coupling static characteristics. This made it possible an analytic solution of the motion and coupling equations and therefor a reduction of the computing power requirements. On the other hand, to limit the hardware cost, we adopted a multiprocessor architecture where the simluation processor work in pipeline.