In this suggestion, I will be suggestion the Köln - Frankfurt Highspeed line as Schnellfahrstrecke Köln-Frankfurt running from Köln Hbf to Frankfurt Hbf via Köln Deutz Messe/Deutz Bf, Siegburg/Bonn and Frankfurt Flughafen. Total distance is 177km or 110 miles but it could be longer if potentially merged with Köln-Aachen. Rolling stock: BR 406 ICE 3 (Upgraded with TVM Displays and different pantograph settings for 25kv ac and Belgian DC lines). Layers/AI/Substitution AI only will mainly be at Frankfurt Hbf and Köln Hbf with some drivable S-Bahn and RE9 layers from Köln-Aachen and New Journeys Expansion including AI TGV Layers at Frankfurt. The BR 403 from HMA and KWZ will also substitute for domestic ICE services. It will be the first German route in TSW3 to have a speed of 300kph or 186 mph.
Schnellfahrstrecke Rhein Main has a unique feature it's the slab track feste Farhbahn instead of the Ballasted track seen on Schnellfahrstrecke Köln Aachen and Schnellfahrstrecke Hannover Kassel Würzburg. Furthermore DB BR403 ICE 3 does combine with DB Baureihe 406 ICE3M which is common on the route.
After Wurzburg to Kassel surely time for another conventional German railway. The high speed line is interesting for a couple of trips either way, but after that gets rather boring especially with all those tunnels and no sound reverb. We need something iconic like the Rhine Valley in TSW.
Even tough SKW is an really cool route, its bit yeah boring sometimes! Like u said; we need something iconic in TSW, or an older route with some older stock or an really busy/icon route!
https://forums.dovetailgames.com/threads/cologne-frankfurt-high-speed-line.27855/ Here you have the perfekt suggestions for Schnellfahrstrecke Köln-Frankfurt