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Previous research has shown that timetables are important for punctuality, and research is ongoing on how to improve timetables through more simulation, optimization techniques, better scheduling of track works, and data analysis. Relatively little attention has been given to the actual planners. In this study, we have conducted interviews with timetable planners in Southern Sweden, and analyzed the interviews with the perspective of explaining Swedish punctuality problems. From these we have identified four common errors in timetables, such as “conflicting train paths at stations” and “insufficient dwell or meet times at stations”. These errors cause, increase and spread delays, and require recurring interventions from traffic control. We also identify 11 reasons for such errors, such as “insufficient time for quality assurance of timetables” and “missing tools for track allocation and conflict management”. We discuss three themes among these reasons: “missing tools and support”, “role conflict”, and “single- rather than double-loop learning”. New tools and processes are currently being rolled out, which is expected to improve the situation with regards to the first of these themes. While the role conflict will remain, the new tools can perhaps also help to elevate the planners from first- to double-loop learning, allowing them to focus on quality control and on finding better rules and heuristics.