Hamburg S-Bahn S3 is a 75km line that runs from Pinneberg to Stade. This line is unique compared to other Hamburg S-Bahn lines, because there's a power change from third rail to overhead wire at Neugraben station. EDIT: This suggestion includes also section between Altona and Pinneberg. Rolling stock: Dual voltage version of DB BR 474
I'd say if the part to Pinneberg could not be included, just create the part Hauptbahnhof-Stade. Why? Maybe you can combine it with the RE Route from Hamburg to Cuxhaven which is operated by a Diesel locomotive (BR 245). Service operator is start (DB subdivision) with ex Metronom rolling stock... They share the most part of the route...
Excellent idea speaking of DB BR474.3 Dovetail Games will try to layer it to other 15kv 16.7hz S Bahn routes München S3 HMA S Bahn Rhein-Rhur S12,S19 SKA S1 & S3 HRR that will be in addition to its assigned DB BR425 DB BR423 & DB BR422. Own this route and the future NEC Haven To New York the DB BR474.3 standard on this route will play like the American Metro-North Railroad M8 due to overhead wire third rail voltage change. The only route to have a third rail to overhead so far is southeast high speed Ebbsfleet Railway Station to st. Pancras