In 1946 the Planning Office for the Central Station was created, led by Egil Sundt. This committee announced an architecture contest for a new central station, and the "Green Light" plan by John Engh won. The planning office started construction based on the 1938 committee's work, and expanded the Alnabru Railway Yard in parallel with the new station. The office presented plans which would take 15 years to implement. One of the greatest obstacles was the location of the tunnel under the city. NSB wanted a straight line for quick transport, but complex geological conditions in Oslo, including large areas of clay, prevented this. The plans originally included a station at Oslo City Hall, but a more northerly line was chosen that would be 1,802 meters long.