This proposed line would create a fast route into Bosnia from the north-west, from Zagreb and west-central Europe. It would diverge from the proposed middle Sava high-speed line from Zagreb to Vinkovci, parallel to the existing main line. At Doboj, it would join the existing main line to Sarajevo.
Bosnia is a difficult place to build railways. It is mountainous, and the river valleys are winding, with gorges in some places. The Sarajevo basin is the only population concentration in the interior. The railway into Sarajevo follows the Bosna valley from Doboj. There are two routes to Doboj itself. Trains from Zagreb follow an indirect route, via Banja Luka. The other route follows the Bosna valley from Doboj to the Sava plain. At Vrpolje, a small junction station, it connects with the Zagreb – Belgrade line.
After the Bosnia war, the rail networks were divided between the new states and entities. The journey from Sarajevo to Zagreb takes over 9 hours, with only two trains a day. The one daily train to Belgrade takes over 10 hours, including a 2-hour wait at Vrpolje.
The route from Belgrade could easily be improved, by a new cut-off line from Vinkovci to Bosanski Šamac. This would shorten the route, and Vinkovci is a more logical junction station.
The route from Zagreb (and western Europe) is more difficult to upgrade. The earlier proposed high-speed line to Banja Luka would shorten the route from Zagreb, but not the winding Banja Luka – Doboj line. The only other option is a second new line, from the Sava high-speed line (HSL). It would start between Nova Kapela and Slavonski Brod, and run toward Derventa and Doboj.
This line would be used by high-speed trains running non-stop from Zagreb, and inter-regional trains from the upgraded Sava main line. (That assumes heavy upgrading of the Doboj – Sarajevo line, but that is not considered further here). The new section would be about 50 km long.
The alignment is not entirely new: there was a narrow-gauge line, from Slavonski Brod to Derventa and Doboj. In fact, this was the main railway into Sarajevo, during the Austro-Hungarian occupation: it was built as a military line in 1879-1882. When the Bosnian narrow-gauge network was converted to standard gauge, in the first and second Yugoslavia, the line was relocated, to the easier route along the Bosna.
A new line could start at Slavonski Brod, but to shorten the route from the Zagreb, it should begin further west. That does not rule out a regional line from Derventa to Slavonski Brod – in effect, a re-opening of the old narrow-gauge line.
There are three topographic obstacles: the Sava river, the Markovac ridge between the Sava and Derventa, and the hills between Derventa and the Bosna valley. Derventa itself is in a wide valley (Ukrina river). With a population of about 15 000. it is large enough to justify a station.
Between the Sava and Derventa, a 5-6 tunnel under the Markovac ridge is the logical option. The line could go around the ridge further east, but that would defeat its purpose. The alignment near Derventa (with station), is dependent on the next section. The old narrow-gauge line reached the Bosna valley without a tunnel, but the alignment was convoluted.
Click to enlarge: The narrow-gauge line from Derventa to the Bosna valley, highlighted in green. The base map is an Austro-Hungarian military map of around 1910.
The new line would pass east of Derventa, on relatively level terrain. It could cross the main ridge in tunnel, and then follow the valley of the Veličanka stream to Kotorsko, like the old line. However, a more direct route with several shorter tunnels might be easier, and the geology is also a factor. Only detailed engineering studies can indicate a preferred route.
In any case, the new line would cross the Bosna near Kotorsko, to join the existing alignment (line from Vrpolje). This line would be upgraded, with a new tunnel under the bend before Doboj. The station is across the river from Doboj itself, but the location is acceptable.
At Doboj, the line would offer interchange with the Banja Luka and Tuzla lines, which would be restructured as an east-west regional route. (That requires a new exit line from Doboj station, toward Tuzla).
The Derventa cut-off line is the ‘maximum improvement’ to the Zagreb – Sarajevo route. The remaining Doboj – Sarajevo section can be upgraded at best, with some cut-off tunnels at river bends. A true high-speed line from Doboj to Sarajevo, would require more than 100 km of flank tunnels, along the Bosna valley.