Hannöversche Südbahn
The Brunswick–Hanover Treaty of 1837 that regulated the construction of the Brunswick–Bad Harzburg railway had already established a basic agreement allowing both countries to work together towards a southern link. Only 15 years later a further treaty was contracted. This allowed Brunswick to build a line from Brunswick to the Hanoverian Southern Railway at Kreiensen. This in turn allowed Hanover to build a railway line from Börßum (in Brunswick) via Ringelheim (Hanover) to Seesen (Brunswick). From there it was agreed to build the line via Harriehausen (again in Hanover) to Kreiensen, where both lines were connected. The line was opened on 5 August 1856. Three months earlier the Hanoverian Southern Railway was opened from Hanover to Kassel.