An Introduction of Two Different Modelica System Integration Concepts

System engineers with a simulative system testing mission are facing the challenge of modeling the system's component-based architecture efficiently and modularly. Therefore, two different modeling concepts are proposed in this paper, which enable building and rebuilding a system modularly with the object-oriented modeling language Modelica. Moreover, the approach is capable of restructuring the model landscape towards co-simulation and Hardware-in-the-Loop (HiL). The concept is tested in a case study on trains.