- ^ Ian Yearsley. Trams are coming back. New Scientist (Reed Business Information Ltd.). 21 December 1972 [2014-01-14].
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.
- ^ John Hoyle. Letters to the editor -- The tram is the answer. Sydney Morning Herald. 16 May 1975 [2014-01-13].
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.