Gaming simulation in the railway sector often uses the same conceptual model as in computer simulation, and enables operators to interact with this model during a simulation run. Therefore, gaming simulation validation poses different challenges. This paper aims to answer the question to what extent gaming simulation can be used as an experimental research setting, due to its loosely demarcated experimental features. Focusing on validity issues, we study five cases in which the Dutch railway

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... tor used gaming simulation to test innovations in a controlled environment. The results show that in addition to traditional external validity issues, human game players inherently open up this controlled environment, bringing in many confounding variables. By signaling what the specific validity threats are, this paper strives to improve gaming simulation for testing innovations that tackle social and technical elements of a system.