There are two rail lines south-east from Zagreb. One is north of the Sava river, through Kutina and Novska, one is south of the river through Sisak and Sunja. A new rail line is planned to connect them, primarily to improve capacity – although this is Pan-European Corridor X, both lines are single-track.
A cross-Sava line between Sisak and Kutina (blue line on the map) is a better option than Sunja – Novska (brown), even if it is more difficult to construct.
The proposed middle Sava high-speed line from Zagreb to Vinkovci, and the associated upgrading of the main line, would provide a 4-track route on the north side of the Sava. A Sisak – Kutina line would integrate better, with services on that line. The Sunja line should also be doubled and upgraded, but primarily as a route into Bosnia (to Banja Luka and Bihac).
The advantage of the Sisak line is the dead straight alignment. It can easily be upgraded for high speed, and high capacity. The railway was built in 1862, and extended to Sunja in 1882. This section curves around the edge of the hills, and the line through Sisak itself is also curved.
Click to enlarge: The lines as originally built, circa 1910, with the approximate alignment of the new line in red, and Sisak – Sunja line highlighted in green. The base is an Austro-Hungarian military map, using some German place names.
In Zagreb itself, the Sisak line has a flat junction with the main line from Rijeka, and another outside the main station. Near the station, there is limited space for extra tracks, which are also essential for new urban services. New rail lines in Zagreb are therefore needed, with central-area tunnels.
Outside Zagreb, the line also serves a developing zone around the airport, including Velika Gorica (population 35 000). A separate light-rail line to the airport is planned, rather than a branch from the Sisak line. There are 3 intermediate stations on the line to Sisak, and 4 halts.
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The alignment into Sisak station is good. The line to Sunja turns south immediately after the station, on a curving alignment through the built-up area.
The line then splits at Sunja. One line continues south-east along the Sava valley. Near Dubica, it turns north across the river, to join the parallel line (Zagreb – Vinkovci). The junction is at Novska station.
The other line from Sunja turns south, and joins the Una valley at Kostajnica. This is the present main rail route to Banja Luka. In 1924 it was extended from Novi Grad to Bihac. From Bihac it was extended to Knin in 1948, connecting with the railway to the coast.
The excellent alignment is a good reason to upgrade the Zagreb – Sisak line, and build a new link across the Sava. Even with a new high-speed line on the northern bank, that would form a valuable parallel route, along the Sava towards Vinkovci and Belgrade. (The Sisak – Sunja alignment is less suitable for high speeds, even if it is doubled).
If the new cross-Sava link begins at Sisak, the line through Sunja will be available for inter-regional passenger and freight traffic, into northern and western Bosnia. With new projects elsewhere, it would no longer be a major transit route. A new Karlovac – Bihac line (part of the Yugoslav-era planning) would offer a shorter route to Bihac. Similarly, the proposed high-speed line to Banja Luka would bypass the existing line, through Novi Grad and Prijedor.
The new link across the Sava can start just outside Sisak station. The station alignment seems to be designed for a future Sava bridge: a freight siding extends east to the river bank. The new line would follow this alignment, along Obrtnička ulica, an industrial area. The line would climb on viaduct, to cross the river Sava on a new bridge.
The alignment would then cross the broad flood plain of the Sava. Much has been reclaimed as agricultural land, but part of it is still flat marshland. The alignment is not specified in detail here. Part of it might be on viaduct: a floodplain is, by definition, at risk of flooding.
On the other side of the floodplain, the new link would join the main line west of Kutina. It would also connect to the proposed Zagreb – Vinkovci high-speed line (HSL).
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The new alignment would be about 28 km long, with a total of about 33 km from Sisak to Kutina, station to station. It could be used by some high-speed services, and should be designed for at least 200 km/h – not a problem on flat terrain.
The line would be used mainly by inter-regional services, in the direction of Vinkovci and Belgrade. These would follow the pattern already proposed, for the upgraded main line. They would stop at Kutina (population 15 000, 79 km from Zagreb along the north bank), at Nova Gradiška (population 16 000, 140 km from Zagreb), and at Slavonski Brod (population 62 000, 190 km from Zagreb), and terminate at Vinkovci.
The route via Sisak is only 5% longer than the main line via Dugo Selo, so perhaps all inter-regional services could use it. In that case, Ivanić-Grad ( 39 km from Zagreb, population 15 000), would have only a regional service.
Combined with more intensive services into Bosnia, an urban-regional service to Velika Gorica, and a possible airport line, the Zagreb – Sisak line would then need 4 tracks, all the way from Zagreb’s main station. The single-track Sisak – Sunja line should be doubled.
The existing line from Sunja to Novska (passing Dubica), would remain in use, primarily for freight. The section across the Sava floodplain could be used for a local service, with a short extension into Dubica town. (The only other village on that line is Jasenovac, with about 800 inhabitants).
There is one more connecting line, or there was: the Sisak – Karlovac line (via Glina). This line through a thinly-populated region was damaged in the Croatian War, and never repaired. It would be more logical to connect Karlovac to Novi Grad / Bosanski Novi, with a new 60-km line from Glina. (In fact that was already planned in the Yugoslav era, at least since the 1950’s). Either way, the Glina line has no impact on the proposed cross-Sava link.