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Why remove RER?
The Berlin S-Bahn also shares its tracks with other rail traffic, as do other S-Bahn systems. Though in the Berlin case that requires double electrification and is thus not done all that often.
No, it doesn't. In the cases you mentioned, it's always four tracks, two for the S-Bahn and two for the mainline railways. All I did was move the RER from the Berlin-Hamburg group to the newer-systems group.
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How well does it match the trope?