The city of Essen was pretty big on testing the system in the 80s. They built sections of different lengths in various areas ranging from just a few hundred meter long ones on inner city streets to multiple kilometers on converted former tram lines in the middle of a highway. They even introduced a line that was partial underground which got served with duo busses (diesel and electric as trolleys) as seen in >>1635466
In the end the system proofed to be unsuitable for the densely populated area. For services on busy sections, even articulated busses are way too small. Aforementioned converted tram line on the highway is being served every 5 minutes nowadays, which makes service very uneconomical. Maintenance is quite expensive too since spare parts are rare. In suburb areas guided bus lines tend to be rather unnecessary. The amount of traffic doesn't justify the costs that come with the system.
Also said underground line got discontinued roughly after just 4 years due to its low reliability.
Today only the highway line remains - and only due to current lack of alternatives. The responsible company Ruhrbahn would like to revert it back into a tramline, but it's quite difficult since regulations for electric rail lines in the middle of highways have been tightened over the years. All other line sections got closed and demolished over the years.