Historically, the rail lines on the Istrian peninsula were oriented toward the Austrian imperial capital Vienna (Wien). The main line ran from Vienna to Trieste, over the pass at Postojna. About 10 south, at Pivka, the line to Rijeka diverges: it runs via Ilirska Bistrica. The main line continues to Divača. There, the Istrian main line diverged, toward the the Austrian naval base at Pula / Pola. About 15 km along that line, at Prešnica, there is a 32-km branch to the port of Koper. It was built in 1967, when the Yugoslav republic of Slovenia developed the port of Koper as a competitor to Trieste. From Divača, the main line continues to Villa Opicina, on the plateau above Trieste. From that station, there are two rail lines down into Trieste itself. The rail connection from Trieste or Koper to Rijeka is therefore very indirect, and requires trains to reverse at Pivka.
The planned Učka tunnel would link Rijeka to the Pula line, near Lupoglav. It is intended to create a Zagreb – Rijeka – Pula corridor, including the planned Zagreb – Rijeka HSL. However it can also shorten the Rijeka – Koper route: trains could reverse at Prešnica instead of Pivka. A cut-off line south of Prešnica would avoid a reversal entirely, and shorten the route further: the options are being studied, but there is apparently no official proposal yet. The costs are justified only by freight traffic, and some Rijeka – Koper passenger traffic. Inland, the only settlement large enough to justify a station is Buzet (municipal population 6 000). Most of the population of Istria is concentrated on the coast, which would be served by the proposed Istria coastal rail line. Passenger traffic between Rijeka and Trieste would best be served by a high-speed line parallel to the Ćićarija range, connecting to the Ljubljana – Venezia HSL.
Neither of the two lines south of Prešnica was planned for Koper – Rijeka traffic. Both are winding and indirect, because they descend from the Kras (Karst) Plateau. Prešnica is at 500 m elevation, and in places the line is built on a cliff. That makes a cut-off line very difficult. The simplest option is a new link from the Pula line, to near Črnotiče. The example shown drops 60 m in 4 km: an acceptable gradient, but it does not shorten the distance much.
If the link is lower down the Koper line, it will be much longer and more difficult to construct. The Koper line at Dol (elevation 200 m), could be connected by tunnel to the small Movraz basin at 170 m, and then by another tunnel to the hillside above Buzet. It could drop to the outskirts of town (around 80 m), but it can not connect to the existing line. Buzet is located on a valley floor (Mirna river system) in a very dissected landscape, and just below the Kras plateau escarpment. The existing rail line (to Pula) is on the escarpment overlooking Buzet, at 400 m elevation. (The vertical axis is exaggerated on the images).
It would probably be simpler, to construct a tunnel from the lowest part of the Koper line, in the Rižana valley, to near Buzet. Both tunnel portals would be at about 90 m elevation, and the tunnel would be about 9-10 km long.
South-east of Buzet, the line must climb anyway, because the Učka tunnel portal will be at 400-500 m. That would require a complete new line, almost as far as Lupoglav (about 15 km). Heavy freight traffic between Rijeka and Koper would be the only justification for that investment – and there is an obvious alternative: by ship. A short cut-off line seems the best option.