Extra rail capacity in Zagreb is a precondition, for new rail lines into the city. That includes the long-planned Ljubljana – Zagreb high-speed line (HSL), and lines proposed here: the middle Sava HSL from Zagreb to Vinkovci, and the Maribor – Zagreb HSL. Although Zagreb is the crossing of Pan-European Corridors X and Vb, only the line to Ljubljana is double-track.
Zagreb lies between the Medvednica hills, and the Sava river. The original Austro-Hungarian railway ran east-west, at the foot of the hills, between Zaprešić and Sesvete. The Rijeka and Sisak lines converge south of the Sava, and then join the east-west line just outside the main station. All trains pass through flat junctions, and because the main station was relocated eastwards, there is an S-bend in the alignment.
Click to enlarge: The railway geography of Agram (Zagreb) around 1910. From an Austro-Hungarian military map, using German place names.
Capacity on the approach lines into the city can be improved by new alignments. In the central area, near the main station (Glavni Kolodvor), they would require some tunnel sections.
New lines can use the topography: a tunnel under the hills can approach the main station from the north – impossible with a surface line. The tunnel would then be aligned with a main north-south road axis, and the Most Slobode (Freedom Bridge) over the Sava. The straight alignment continues to the Sisak rail line. This tunnel would create a new west-to-southeast route (orange line on the map), which has no equivalent in the existing rail lines. It requires new underground platforms at the station, but there is enough space under the square in front(Trg kralja Tomislava).
This Most Slobode alignment can be linked to the Karlovac line, with a new line (green on the map), alongside the Zagreb bypass motorway. The service pattern would resemble the the Pilzkonzept or ‘mushroom pattern’ planned for Berlin Hauptbahnhof. (This pattern is also suitable for a possible regional metro line).
An alternative exit route to the Sisak line, is along Avenija Marina Držića, shown in blue. This is another wide road, leading to a Sava bridge (Most Mladosti). It continues as Sarajevska cesta, but the last section of this road is incomplete. The new rail line would in any case enter a tunnel, to pass under the marshaling yard, and join the Sisak line at Veliko Polje. It could also link to an airport rail line (brown line).
This alignment requires a curving exit tunnel passing under the main bus station (shown in red on the image below). The western access to the main station could be improved, by relocating the Zaprešić line (red/black) southwards, crossing the Karlovac line. That would require only limited demolition, and would connect to new platforms (south of the existing platforms).
Click to enlarge…
With a new exit to the Sisak line, the existing line from across the Sava can be reserved for traffic from the Karlovac line. The rail bridge, and the curve just north of it, must be improved. Upgrading of this route is essential anyway, for the planned high-speed line to Rijeka.
Regional services from the Karlovac line could use the suggested Most Slobode alignment, to reach new terminal platforms under the main station.
The lines suggested here are primarily for long-distance traffic. They are not generally suitable for urban-regional services (S-Bahn). The existing east-west line would be suited for an intensive service, but it does not have the spare capacity. A possible solution is to build a parallel metro line, and reserve the existing rail alignment for non-urban trains. The former bypass road (Slavonska Avenija) is also a possible alignment for a regional metro.
Rail lines through central Zagreb should be reserved for passenger traffic. That requires a good freight bypass, for instance along the bypass motorway. In any case, a new ring motorway is planned further from the centre, with a new freight bypass. In combination with the existing bypass lines, it could remove all through freight from central Zagreb.