A week and a half ago, I crayoned Berlin U- and S-Bahn expansion on video. With some tweaks, here is the final product:
Here is the full-size version. (I know I’ve been asked to provide lighter JPGs, but my attempt at JPG compression turned 86 MB to 37 MB, hardly a coup de grâce.)
This is based on ongoing U-Bahn expansion plans plus the 2030 S-Bahn plan.
The most significant variation is that the dashed S-Bahn line from Gesundbrunnen to Hauptbahnhof and Potsdamer Platz, dubbed S21, is turned into a northwest-southeast trunk line in my plan, following a proposal by Felix Thoma in Zukunft Mobilität. The plan for S21 today is to stay north-south and link with Südkreuz and Schöneberg, beefing up frequency on the north-south S-Bahn.
I believe my routing to be superior, due to traffic on the Görlitzer Bahn, seen below (source, p. 6):
Currently, peak traffic on both the Stadtbahn and the North-South Tunnel is 18 trains per hour in each direction. This is low; Munich achieves 30 tph with very short signal blocks and more branching than Berlin has, splitting into seven branches on each direction rather than three or four. 30 is a limit value, but 24 is more common, and would substantially simplify operations.
The North-South Tunnel splits into a western branch, currently carrying S1 via Schöneberg to Wannsee every 10 minutes, and an eastern, carrying S2/S25/S26 via Südkreuz every 10/20/20 minutes; since the two branches have roughly equal ridership, each should run every 5 minutes, unlike today, where only Südkreuz gets such service. To the north, each of the two main branches can run every 5 minutes as well.
The Stadtbahn is asymmetric. Only 12 out of 18 tph continue west of Westkreuz: Spandau and Potsdam get 10-minute service, and in addition S5, turning at Westkreuz, runs every 10 minutes. As such, all growth in traffic on the western branches should be encouraged. This is thankfully already done, with expansion plans west of Spandau. To the east, traffic is the most overloaded, and will remain so even with the opening of the U5 extension last year. Going up from 18 to 24 maximum tph means 10-minute service on each of the four branches – S3 to Erkner, S5 to Strausberg-Nord, S7 to Ahrensfelde, S75 to Wartenberg (proposed to be extended into a loop going northwest). Today, S3 runs every 20 minutes, and S75 doesn’t run through but rather only runs from Warschauer Strasse east, and conversely, S9 curves from the Stadtbahn to the Görlitzer Bahn to the airport.
Rerouting S21 to connect to the Görlitzer Bahn means that trunk, currently carrying 18 trains per hour, can all run through to city center, and then either go to the Siemensbahn or loop from Hauptbahnhof to Gesundbrunnen. Such service also removes reverse-branching from the rest of the system, allowing all services to run more regularly and reliably since each of the four trunks, including the Ring, would run independently of the others, and delays wouldn’t propagate.
U-Bahn expansion in Berlin is mostly mothballed. The city prefers trams, even where they are inappropriate due to low speed over long stretches or forced transfers. Plans for U-Bahn expansion to Märkisches Viertel are uncertain, unfortunately. Plans for expansion to Tegel along a branch of U6 look dead, hence my resurrection of an older unbranched U5 extension; the current plan is to connect the Urban Tech Republic complex with the rest of the city via tram. Trams are cheaper but you get what you pay for; the ideal use of a tram is for cross-city routes, not primary routes to the center.
Hence various extensions that I think should be built. U7 to the airport looks like a done deal, and U7 to Staaken is favorable too, as is the low-cost, low-ridership one-stop extension of U3 to Mexikoplatz. U9 to Pankow and U2 to Pankow-Kirche are much-discussed, as is U8 to Märkisches Viertel, whose current cost/rider projection is favorable by international standards.
My additions are U1 extensions at both ends, the U5 extension to Tegel and then looping to intersect U6 and U8 in Reinickendorf, and the resurrection of the U10 plan as a U3 link (and not as a line to Steglitz, which gets extra S-Bahn service either way). The U1 extension to the west is forced to use cut-and-cover since the U1 tunnel under Kurfürstendamm is 1900s cut-and-cover, which is disruptive but cheaper than bored tunnel. The other two lines are long-term desires of the city and have been safeguarded for decades, with intersecting stations built to accommodate them.
Whether lines run in this configuration or another is up for debate. At Wittenbergplatz it’s easiest to link the new U10 system to U1 to Uhlandstrasse and then connect U3 to Krumme Linke with the existing Warschauer Strasse terminus. This would be an awkward system of U1, U2, and U3 in which the line going farthest north going east also goes farthest north going west and the line going farthest south to the east goes farthest south also to the west. If there’s a way to flip the situation, pairing U10 with present-day U1-west, U2-east with U3-west, and U3-east with U2-west, it should be done; this system in general has undergone many such changes over the generations.