After the death of Hermann Bachstein in 1908, the Rhenish industrialist Hugo Stinnes took over the majority of SEG in 1909 with the aim of taking control of the electric tramways of the Ruhr and other major cities as part of the RWE (Rheinisch-Westfälisches Elektrizitätswerk AG) power company so that they could be connected to the electric grid of western Germany.

In order to focus on electric trams, a few railways were spun off the SEG and transferred to newly established companies.

In order to carry out the electrification of the line and to allow the expansion of its tram network, the city of Mannheim in cooperation with the SEG, which had acquired the Mannheim-Weinheim-Heidelberg-Mannheim railway, founded the Oberrheinische Eisenbahn-Gesellschaft AG (OEG) in 1911.

In order to prevent domination by the RWE, the city of Mannheim held a majority of the shares of the company.

[2] To mitigate financial problems after the First World War, the city of Mannheim took over the railways from the OEG, while the company (with retained power plants and electric distribution) remained in the hands of Stinnes and was renamed Kraftwerke Rheinau.

At the time of the line's construction, Rheinau was not yet incorporated in Mannheim and was therefore in the jurisdiction of the OEG according to the existing cooperation agreements.

Since the construction could not be completed even after the war for economic reasons, the OEG began operating its first bus line on the route in 1925.

Like most transport companies in Germany, the OEG had a huge increase in ridership after the war to cope with, especially in 1945 when only half of its fleet was available.

One of the reasons for the increase was the phenomenon of so-called "foraging trips", when cheap fares encouraged people to barter for food with farmers.

About 1960, it began improving the technical safety of level crossings with flashing lights, which were later augmented with half-barriers.

Rhine-Neckar Transport (Rhein-Neckar-Verkehr GmbH, RNV), which is jointly owned by MVV OEG AG, MVV Verkehr AG, Straßenbahn Heidelberg, Verkehrsbetriebe Ludwigshafen and Rhein-Haardtbahn, was founded on 1 October 2004 and it has carried out joint operations since 1 March 2005.

Since 2005, responsibility for funding losses incurred has been allocated to the respective districts of Mannheim, Heidelberg, Viernheim and the Rhein-Neckar-Kreis in proportion to net-kilometres operated.

The merger was finalised on 16 March 2010 with the deletion of MVV OEG AG from the share company register.

Furthermore, some passenger cars and numerous freight wagons have been preserved at, for example, DEV Bruchhausen-Vilsen, the Selfkantbahn, the Märkische Museum Railway at Plettenberg, the Albbähnle (Amstetten–Oppingen) and the Härtsfeld-Museumsbahn in Neresheim.

The previous owner had converted the locomotive from a steam tram engine, but it had become redundant as a result of the closure of the tramway between Utrecht and Zeist in 1949.

Unly the AEG engines, the scrapped locomotive's maximum current switch and wheelsets could be reused in the rebuild.

Towards the end of the 1950s, the bodies of railcars 4 and 17 were rebuilt in the form of the then current vehicles by Waggonfabrik Rastatt, but leaving the electrical equipment largely unchanged.

Set 27/28 was preserved as a half train in the Hannoversches Straßenbahn-Museum and serves as an example of the smooth transition from tramway to railway.

Some vehicles of this type were equipped in the first half of the 1970s with magnetic rail brakes and low voltage systems, brake lights, turn signals and validators, and later also inductive point control and the Punktförmige Zugbeeinflussung (Indusi) train control system was installed.

The car remained unique and were subjected to multiple modifications over time (gearboxes, doors, pantographs, validators, Indusi, signalling control).

The 13 railcars (98 to 110) supplied in 1973/1974 were no longer equipped with partitions, luggage nets and blinds on the windows and were not designed for 750 volt contact wire tension only as built.

The Duewag Gt8 sets of the different builds could be coupled with each other by means of Scharfenberg couplers to form train assemblies.

The Gt8 dominated the appearance of the OEG for decades and have been continually modernised over the years (one-man operation, spring brakes, driver's desks, passenger seats, microprocessor-monitored control panel controls, exterior and interior lighting, outdoor LCD destination displays, indoor stop displays, colour scheme, automatic vehicle location system).

Since these were one-directional vehicles, they could not be used on the circuit, but only operated on the Mannheim Hbf–Käfertal–Heddesheim line, since there were turning loops only at the end points.

After the inductive power transmission system was trialed by the manufacturer Bombardier in Augsburg, the car was used by the RNV as a test and training vehicle and equipped accordingly.

So as not to have to fundamentally change all documents and vehicle files, the former operating numbers were simply preceded by a fourth digit.

However, due to the suspension of OEG bus operations in December 2009, the extended numbers were never attached to the outsides of the buses.

The OEG was one of the first non-federal railways in Germany that had vending machines at its stations as a prerequisite for one-man operations.

Between 1980 and 1990, with the conversion of the routes in the city networks of Mannheim and Heidelberg from 600 to 750 volts, the running voltage was standardised for the first time.

The reversal of polarity at the entrance and exit to the Heidelberg network was carried out automatically over approximately 70 metre-long, neutral sections, which were run over using momentum.