Analysis of information sources in references of the Wikipedia article "Stadtbahn" in English language version.
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But instead of building the entire expensive systems immediately, the Germans hit on the idea of building only the city centre tunnels at first. Intended in the long run to be extended to full undergrounds, in the short term they could be used by trams which would continue to run on the surface outside city centres.
Cities such as Frankfurt and Cologne in West Germany have further developed their tramway system by introducing a concept known as "premetro." In this system trams or light rail vehicles make extensive use of tunnels, reserve track and by utilizing folding steps these vehicles can operate through high or low stopping places.
When I talk about 'Stadtbahn' in this context, I mean those systems which from the late 1960s started to build underground sections to full metro standard, and with the final goal to converting these to full metro operation (like a pre-metro). As we know, none of them actually achieved this initial goal, but all gave up sooner or later.
But instead of building the entire expensive systems immediately, the Germans hit on the idea of building only the city centre tunnels at first. Intended in the long run to be extended to full undergrounds, in the short term they could be used by trams which would continue to run on the surface outside city centres.
