Advances in digital technologies over the past decade have made Digital Twins an important ingredient in the development of new products and tracking their performance over the lifecycle. Nonetheless, the definition and structure of Digital Twins are still under debate. The SAPPhIRE model initially developed to represent the causality of a system offer significant advantages, such as the product behavior and its evolution, and most importantly, how different parts of the system interact. In this paper, the concept of representation of the virtual part of a Digital Twin based on the SAPPhIRE model is proposed as a solution for defining and developing the Digital Twin. The synergy between the Digital Twin model and the extended SAPPhIRE model provides an opportunity to conveniently build the virtual part of the Digital Twin. To substantiate the concept proposed in this paper, a Digital Twin of a train braking system is demonstrated.