Equipment models used in fab-level simulation do not typically include features such as internal wafer buffers and setups that depend on wafer locations inside the tool. Such features are especially important to system performance in the presence of smaller lot sizes and greater product diversity. In this paper, we provide an introduction to flow line models that allow one to incorporate these key practical behaviors. We then develop flow line models for clustered photolithography tools and conduct simulations to assess the quality of the models. Despite the fact that the models only incorporate wafer transport robots via a constant addition to the process time, they can be quite accurate. When tested against data from a clustered photolithography tool in production, the model predictions for throughput and cycle time were within 1% and 4%, respectively. The computational requirements are about one order of magnitude less than is otherwise possible.