The railway lines along the Rhine are among the oldest and most strategic in Germany. South of Köln (Cologne), they pass through the Rhine Gorge (from Koblenz to Bingen). There are double-track electrified lines on both banks: the right-bank Rechte Rheinstrecke and the left-bank Linke Rheinstrecke.
The right-bank line carries most freight, the left-bank line all long-distance trains. All passenger trains pass through Koblenz Hauptbahnhof, crossing the river if necessary. The lines were always a bottleneck. Because of the topography space is limited, there are historic buildings adjoining the line, there are parallel main roads, and the alignment is inevitably curved.
River, road and rail at Lorch: image under CC3.0 licence by Traveler100.
For that reason, the new high-speed line Köln – Frankfurt avoids the Rhine valley entirely: it follows Autobahn A3 across the plateau, east of the river. Upgrading of the existing alignments is difficult, and therefore new flank tunnels should be considered.
Flank tunnels and cut-off tunnels
In mountainous regions, a rail tunnel normally connects one valley to another: the valleys are the preferred route, because construction is easier. Sometimes the valley floor is so narrow or winding, or the valley sides too steep or unstable, to permit construction. A tunnel in the flank (side) of the valley is a solution: it can be used to replace an older line. In meandering valleys, tunnels are also used to shorten the route, by avoiding a meander. They typically cut through the neck of the meander. In the Rhine gorge, a combination of flank tunnels and cut-off tunnels is appropriate.
Although it is theoretically possible to construct separate flank tunnels on both sides of the Rhine Gorge, the alignment proposed here crosses both existing lines anyway. It is therefore simpler to have one new alignment, connected to both existing lines. In total three new bridges are required. Apart from that, the proposed new line is entirely in tunnel.
Why upgrade the Rhine Gorge line?
There are six tracks available between the Rhein-Ruhr and Rhein-Main metropolitan regions. Given the strategic importance of the Rhine valley route, that is probably not enough. New flank tunnels in the Rhine gorge would increase capacity, although not as much as a complete parallel line. They would cut journey times for long-distance trains, especially by avoiding the meander at Boppard. They would have environmental benefits: the gorge is not only a busy transport corridor, it is a World Heritage Site.
The alignment proposed here is schematic: it avoids the most curved sections of existing line. On the left bank it would run from Rhens to Oberwesel, and on the right bank from Braubach to north of Kaub. It would be about 20 km long, about 10 km less than the existing lines. (Detailed studies would be needed before any alignment was finalised).
The new line from Rhens would turn into the flank, and rise in tunnel to cross the river by bridge. Rhine bridges have a standard clearance of 9 m, but a higher bridge would minimise conflict with the infrastructure along the banks. The bridge would be at the south end of Oberspay (approximately where the cruise ship is on this image). On the opposite bank, the new line would directly enter a tunnel, at the bend opposite Oberspay. Both tunnels would link up, underground, about 2 km from the bridge.
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The line would emerge from tunnel at Ehrentahl, and cross the river at right angles. The channel has been regulated here, and there is small island in the river. On the other bank, the line would go straight into a tunnel again, to continue southwards. The tunnel would pass west of Sankt Goar, and then split.
One branch would turn west to the exit portal on the left bank, at Oberwesel, just north of the historic tower. If there is insufficient space here, for a grade-separated junction with the existing line, there is an alternative portal site at the south end of Oberwesel.
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The other branch would emerge from tunnel, to cross the Rhine east of Oberwesel. On the other side, it would again enter a tunnel. (The bridge site is behind the sandbank on this photo looking east). The line would then turn to the river bank, to a tunnel portal north of Kaub, approximately opposite Schönburg castle.
There would be no intermediate stations. To maximise use by both passenger and freight trains, the line would have a standard operating speed of about 120 km/h. All junctions would be grade-separated.
The current long-distance services south of Koblenz are the ICE lines 31 and 9, and the IC lines 30, 31, and 32 (all at two-hourly frequency), with a few extra trains from IC lines 35 and 55. All run to Mainz: a few IC trains also stop at Bingen. All these trains, and their successors, can use the new tunnel. (One of the justifications for the tunnel is an increase in the frequency of these services, and a more regular pattern).
There is a Regional-Express service on the left-bank line, the RE2 Mittelrhein-Main-Express Koblenz – Boppard – Oberwesel – Bingen – Mainz – Frankfurt (via Frankfurt Airport), every two hours. The regional services are the SE10 Rheingau line (right bank) Koblenz – Wiesbaden -Frankfurt, and RB 32 Mittelrheinbahn (left bank) Koblenz – Bingen – Mainz, both hourly. These local lines would not use the tunnel, but it would allow an additional express service, between Koblenz and Bingen.