Ever since the introduction of the railway through Stockholm there have been issues regardingcapacity. Citybanan is a recently built railway tunnel for commuter trains that takes somepressure off the heavily frequented stretch of tracks in the center of Stockholm. The projecttook almost 30 years to complete and led to time delays as well as large cost increases. Usinga theoretic background of path dependence and megaproject theories the planning is beingexamined. The conclusions of this study are that the investigations are influenced by pathdependence, mainly because the Swedish government early on decided upon reserving moneyto a specific project which did not turn out to be the technically best project. This pathdependence is related to the megaproject theory that projects that lock onto a specific idea oftenget delayed. The overrun might have been prevented by comparing Citybanan to other finishedrailway projects during early stages of the investigation.