This paper describes an applied simulation study on the flow of rail replacement buses in Oslo. We performed the studies using SIMADES, a newly developed multi-agent discrete event simulator. In the project, we enriched the simulator with agents representing entities commonly found in the urban mobility domain. Our study identified the main bottlenecks that limit the flow of buses through the road network under the traditional way of scheduling and dispatching buses. The study furthermore identified the best actions to implement that enable a significant increase of capacity for rail replacement buses. The conclusions are expected to lead to concrete changes in how the rail replacement bus service will be executed in Oslo during planned and unplanned maintenance of the rail network.