This paper addresses a significant challenge experienced by infrastructure managers concerninghandling and scheduling slot requests for additional trains, or urgent track maintenance,after the master timetable has been finalized. In congested railway networks, wherepassenger trains share the tracks with freight trains and where the freight train operatorscannot fully predict the actual need for access to train slots long in advance, there is aneed for a flexible and effective timetabling revision process. Since the re-scheduling oftraffic and maintenance is a demanding task, the benefits of using computational schedulingsupport is evident. From the perspective of an infrastructure manager, we propose andexperimentally evaluate an optimization-based approach for assessment and scheduling ofadditional slot requests. When inserting several trains, the relations between time and routeoverlap as well as direction of trains, and the required computation time are investigated.The optimization-based approach relies on a Mixed Integer Linear Programming formulation.In this model, the explicit capacity restrictions of line segments and station tracks,including track and platform length, are considered. This model also permits bidirectionaltraffic on all lines where relevant. The experimental results show that optimal solutions canbe retrieved quickly in many scenarios, while for certain scenarios the proposed approachis too time-consuming. The required computation time is very dependent on the propertiesof the inserted train and maintenance slots, respectively.