[8] The dense tram network in the city center that had been present before World War II was only partially rebuilt afterwards, and the Hauptbahnhof (Central Station) was not linked to important places like Neumarkt and Heumarkt for the time being.

This meant a design similar to overground trams with small turning radii, short distances between stops and low-floor platforms.

[10] When construction of the first tunnel started in 1963, there was no promise of the state NRW or the federal government to finance the project, and in the worst case the city would have had to pay completely on their own.

[13] The integration of the Rheinuferbahn and Vorgebirgsbahn was already planned in the mid-1960s, but had to be put off due to a lack of vehicles that could run on interregional rail and partially tunneled Stadtbahn.

To allow the introduction of low-floor trains without having to demolish existing high platforms, the network was divided into two sub-networks of different floor heights.

While there are some stretches that can be described as classic tramway lines, the majority of the east–west-network has been upgraded to a high standard with long sections having segregated right-of-way, justifying the term "low-floor light rail" (Niederflurstadtbahn).

[21] When it became clear to city authorities that the construction of a large number of additional high platforms was not financially feasible, other options were investigated.

[21] The high-floor network consists of all lines operated by "classic" light rail vehicles with a floor height of roughly one meter above street level.

This tunnel was considered a priority because Neusser Straße is a relatively narrow main street and was the most used part of the network outside the city center at the time.

At the station Marienburg (since renamed to Heinrich-Lübke-Ufer) it diverges from its previous routing to the center of Rodenkirchen to the Rheinuferbahn of the Köln-Bonner Eisenbahnen (Cologne-Bonn railways) which it follows to Bonn.

At Friesenplatz it crosses under the Innenstadttunnel which forms the currently (2022) only completely underground station junction which is a bit messy because of the stacked side-platforms.

[26] The station Christophstraße/Mediapark was the location of the worst accident in the recent history of the Stadtbahn in 1999, when a CitySprinter prototype hit a standing train at 50 km/h (31 mph).

This produced an uncommon problem: Because the routing passed the past headquarters of RTL, shaking, which could've disrupted the broadcasts, had to be avoided using a complex damping system.

This connection was supposed to cross the Militärringstraße on a bridge and the run through a green space to the Autobahn A1 where the line would've ended in a turning loop.

The planned tunnel was not deemed financeable by NRW and so the city council decided in 2006 that a routing along the surface, that only passed along the edge of the residential area, would be done.

The first stage aims at fixing these problems, by rerouting line 16 from the Innenstadttunnel to the newly built one and building new stations directly under the old city center.

[41] The third stage aims to provide a better service for the residents of the Südstadt, Bayenthal, Marienburg and Raderberg by extending line 5, decreasing the travel time from the end of A 555 to the Hauptbahnhof from 30 minutes to just 13.

To avoid the possible destruction of archaeological artefacts underneath one of the oldest cities in Germany, the NSS was bored down to nearly 30 m (98 ft) below ground level.

This was due to the lower cost than the tunnel, the disadvantages of at-grade crossings, the about 700 m (770 yd) shorter route, the complete replacement of bus line 132 which currently takes 44 minutes to complete the journey into the city center, the smaller impact on the green spaces and adjacent neighborhood and because this route was intended in the planning of the development area Rondorf Nord-West removing the need to change its plans.

[57] In Meschenich Park and Ride facilities are planned on both ends of the town and the Stadtbahn is supposed to operate on the street where traffic will be reduced by a bypass that's already in construction.

[66] Because parts of the network were built before accessibility was a concern, many older stations don't feature elevators/ramps to get from street level to the platform without steps.

[68] Lines 4, 13 and 18 will have in total 39 platforms upgraded to accommodate new 70 m (230 ft) long trains and add sufficient fire protection for the increasing number of passengers in underground stations.

Since the last traditional tramcars were retired in 2006, the Stadtbahn's active rolling stock consists exclusively of modern articulated light rail vehicles.

[87] The cause was determined to be a technical failure by a component beneath the steps,[88] and later narrowed down to an overheated power converter responsible for the breaks.

By the time a new series of high-floor vehicles was needed, Duewag had been sold to Siemens Transportation Systems and later dissolved, meaning that additional Type B cars were no longer being made.

The trains will feature more efficient air conditioning than previous ones, better passenger information screens and sensors that enable preventive maintenance.

[68] For the first time in the system's history, in 2021 a design mockup was constructed at tram museum Thielenbruch used to get feedback on accessibility and the layout of the driver's cab.

Only a year later the current Betriebshof West in Scheidtweiler Straße was opened, keeping the wooden halls from the old site which later burnt down due to bombing during World War 2.

This new parking space was not needed technically, but it was built anyways to attract potential new personnel from Bergisch Gladbach by providing a close by depot.

[100] In 1927 a depot was opened at Hermeskeiler Platz where now the terminus of line 9 is located, and for a short time it was the largest Betriebshof on European mainland.