Petržalka, the only suburb of Bratislava south of the Danube, is the present terminus of rail services from Vienna. From Petržalka Station, trains can reach the main station Bratislava Hlavná Stanica, but only by a circuitous route through the western suburbs. A direct tunnel would be much shorter, and could possibly be combined with a tunnel for urban-regional services (S-Bahn) in metropolitan Bratislava. This proposal assumes the construction of a high-speed rail line Vienna – Bratislava, but a tunnel could be constructed separately. The map below shows the metropolitan context: in black are Petržalka and Hlavná Stanica stations, and the lines approaching them from the north and the south. The existing link line is shown in dark green.
A tunnel is not needed for Vienna – Bratislava – Budapest services, because the existing lines are sufficient. The historical route from Vienna crosses the Marchfeld plain, crosses the Morava to Devínska Nová Ves, and turns south to Bratislava. It curves to avoid the southern tip of the Carpathian mountain range: Bratislava station is located at the tip of the curve. East of Bratislava, the Budapest line crosses the plains, to the Danube Bend. The other main Vienna – Budapest route, the Wien-Raaber Bahn via Bruck an der Leitha, avoids Bratislava entirely. The main use of a north-south tunnel would be for north-west to south-east long-distance trains (Brno – Budapest), and by the line from Vienna as far as the main station. Vienna – Bratislava – Budapest trains would continue to use the Marchegg line.
The tunnel for long-distance trains would therefore run from Petržalka Station, to Bratislava Hlavná Stanica. The main issues are what route it should take, and whether there should be an intermediate station. The shortest alignment for a direct tunnel – the western alignment – is shown as a red line (with north on the left). The tunnel profile is determined by the height of the northern portal, probably on the tracks north of Hlavná Stanica, at about 175 m. The tunnel must also cross the Danube: the river bank is at about 130 m, the ground in Petržalka about 135 m. The southern portal would be near Rosovská Cesta, so the tunnel would be at least 3500 m long.
Most of the 2 km on the northern side would be needed for the climb to Hlavná Stanica, on a gradient of more than 3%. The historic city centre is at about 140-150 m altitude, so any station would be about 40 m deep. These problems seem to rule out a station under the city centre. (A rail bridge would be even more difficult: a tunnel would be needed anyway, from the bridge to Hlavná Stanica).
The northern station would be alongside Hlavná Stanica – close enough to operate as a single station. It would be at street level, or just below it. From there, the tunnel would approximately follow the main road south (Štefánikova), passing near (or under) the Presidential Palace. Continuing this line, it would pass under the old town, almost on the line of the streets Michalská and Venturska.
The section under the Danube would be about 200 m east of the Nový Most (New Bridge). It would then pass west of the shopping centre Aupark (the largest in the city). Still in tunnel, the line would pass under the Panonska Cesta (the road to the bridge), and then emerge from tunnel to join the existing rail line north of Petržalka Station (at the Rosovská Cesta overbridge).
The depth profile would allow for a relatively high speed in the tunnel: trains would accelerate downwards from the stations, and use the uphill gradient for faster braking. Just 4 minutes, from station to station, is a realistic time. The alignment, profile, and journey time would however be different, if the main user of the tunnel was an urban-regional metro (S-Bahn). Because of the incompatibilities, the simplest option is to build a separate north-south tunnel, for the urban-regional metro.