A fourth tunnel, to be used for the D line "(Goethestraße - Sallstraße)", has been proposed but has not been realised so far due to the high costs of construction, currently estimated to be around € 1 billion. Nevertheless, the above-ground parts of the D line, most notably the "D Süd", connecting the Hanover fairground to the C tunnel, have been upgraded to proper Stadtbahn standards, some preparations for interchange with the current stations, such as an empty station below the current Hauptbahnhof station, have also been built in the past.