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The S28 is the only "private" S-Bahn line in Northrhine-Westphalia and except for the Breisgau and Ortenau S-Bahnen the only privately operated S-Bahn in Germany until December 2010. "Private" railways in Germany are not operated by Deutsche Bahn, but are instead owned by state and local governments.
Regiobahn was incorporated on 8 August 1992 by its constituent cities and districts. The company is owned as follows:
- 35 %: city of Düsseldorf
- 20.0 %: city of Mettmann
- 11.8 %: Rhein-Kreis Neuss
- 11.6 %: Stadtwerke Neuss (the public transport operator and supplier of natural gas, district heating and water of the city of Neuss)
- 11.6 %: city of Kaarst
- 10.0 %: Stadtwerke Wuppertal (the public transport operator and supplier of electricity, natural gas, district heating and water of the city of Wuppertal)
From 24 September 1998, it renovated the western section and operated it as a regional line. Passenger services continued to be operated on the eastern section until 2 January 1999. The foundation stone for the new service was laid at Mettmann station on 13 March 1999.
On 26 September 1999 operations commenced on the route between Mettmann and Kaarst initially running every hour and from 28 May 2000, running at 20-minute intervals.
The line actually starts in Kaarst at the Kaarster See station ("Lake of Kaarst") and follows the remaining section of the Neuss-Viersen railway until Neuss Hauptbahnhof. The line runs along the Mönchengladbach–Düsseldorf railway to Düsseldorf Hauptbahnhof and then along the Düsseldorf–Elberfeld railway to Düsseldorf-Gerresheim. At Neuss, Düsseldorf Hauptbahnhof and Düsseldorf-Gerresheim there are connections to the lines S1, S6, S5/S8, S11 and S68 (in peak hours), which runs into the direction of Solingen, Dortmund, Essen, Cologne, Mönchengladbach, Hagen, Bergisch Gladbach and Düsseldorf Airport. From Düsseldorf-Gerresheim to Mettmann the line runs along a remaining part of the largely closed Düsseldorf-Derendorf–Dortmund Süd railway through Neandertal and terminates in Mettmann-Stadtwald, which together with Mettmann-Jubiläumsplatz bus stop is an important traffic node. Services are operated with Bombardier Talent diesel multiple units.