The rail line from Aachen to Köln is an international route, recently upgraded for the Thalys to Paris.
It is officially a high-speed line, Schnellfahrstrecke, but only the 39-km section Düren – Köln permits speeds of over 200 km/h. It has separate tracks for S-Bahn services (with some 3-track sections). The double-track Aachen – Düren section (31 km) has yet to be fully upgraded, and it is shared by regional services and freight. The maximum speed is 160 km/h or 140 km/h (less on some sections).
East of Aachen, the line runs at the northern edge of the Eifel plateau. The alignment here, through Stolberg and Eschweiler, can not be upgraded for very high speeds. A bypass line would avoid these curving sections, and can be extended past Düren, to shorten the entire route. The Autobahn A4 is the only logical alignment for a new line. North of the Autobahn is a large zone with opencast coal mines, the Rheinisches Braunkohlerevier.
Rheinisches Braunkohlerevier, Thomas Römer/OpenStreetMap data CC 2.0 licence.
The by-pass line would diverge from the existing line out of Aachen, east of Eilendorf station. It would cross the A44 autobahn, south of Kreuz Aachen, and turn north-east to the A4 Autobahn. (This alignment is flexible, the area between the A44 and A4 is mainly forest).
The new line would then follow the A4 north of Eschweiler. The main Köln – Aachen line runs close to the edge of the hills. The old Mönchengladbach – Stolberg line runs parallel, north of the main line. It is now used by the regional euregiobahn services, releasing capacity on the main line. In 2009, it was connected to the main line by a new 2,4 km link line to Langerwehe, so that euregio trains can continue to Düren. These alignments would remain in use: the new line is intended to bypass Eschweiler. It would follow the A4 closely, on its south side, away from future mining operations.
East of Weisweiler, the existing line runs in a straight line into Düren. The new line can rejoin it near Langerwehe, using an alignment parallel to the 2009 link line (shown in red). Possibly, it could join the existing line east of Langerwehe (green line). The most logical option is however, to extend the line further along the A4. With 93 000 inhabitants, Düren is large enough to justify service by some ICE trains, but a Paris – Köln route also justifies trains which bypass it.
Again the alignment would simply follow the A4. North of Arnoldsweiler it would turn away from the Autobahn, to rejoin the existing line north of Merzenich. The Hambach opencast mine is being extended south to the railway, and the A4 relocated, but that has no consequences for the alignment proposed here.
The new alignment would then be approximately 28 km long, with no intermediate stations. It would be used only by high-speed trains on the route Aachen-Köln, including possibly inter-regional express services.