The official project for a new line from Divača to Koper is part of a larger project for a high-speed line from Ljubljana to Trieste and the Po plain. The alignment is shown in blue on this summary with map: Nova železniška povezava Trst – Divača. There is a much more detailed technical study available, in Slovenian and English, with detailed maps and artists impressions of the new line:
The line must run in semicircles, because it must descend from the Kras (Karst) Plateau at 450-500 m, to sea level. The normal advantage of a tunnel route – it can run in a straight line – does not apply. With the proposed gradient of 17 per 1000, the tunnel can not be shorter than 26 km – although the plateau is less then 10 km from the port zone.
The same problem applied to the old Austrian line into Trieste (which zig-zags along the plateau edge), and the the existing Yugoslav-era line into Koper, which uses valley-side alignments to descend from the plateau. The planned line to Koper has no consequences for the proposed rail line along the Istrian north coast to Piran, and its extension along the west coast to Pula. They can be built as entirely separate projects, but they are complementary. The new Divača – Koper line would shorten journey times from Ljubljana to Koper, and allow better connections there with the proposed Istria coast line.