A seemingly simple, yet widely applicable subroutine in automated train scheduling is the insertion of a new train path to a timetable in a railway network. We believe it to be the first step towards a new train-rerouting framework in case of large disturbances or maintenance works. Other applications include handling ad-hoc requests and modifying train paths upon request from railway undertakings. We propose a fast and scalable path-insertion algorithm based on dynamic programming that is able to output multiple suitable paths. Our algorithm uses macroscopic data and can run on railway networks with any number of tracks. We apply the algorithm on the line from Göteborg Sävenäs to the Norwegian border at Kornsjö. For a time window of seven hours, we obtain eight suitable paths for a freight train within 0.3 seconds after preprocessing.