The subways of Tolouse (France) and Turin (Italy), and several others, are based on that concept, JFYI: https://en.wikipedia.org/wiki/Turin_Metro https://en.wik

The subways of Tolouse (France) and Turin (Italy), and several others, are based on that concept, JFYI: https://en.wikipedia.org/wiki/Turin_Metro https://en.wikipedia.org/wiki/Turin_Metro https://en.wikipedia.org/wiki/Toulouse_Metro https://en.wikipedia.org/wiki/Toulouse_Metro https://en.wikipedia.org/wiki/Toulouse_Metro#Technology https://en.wikipedia.org/wiki/Toulouse_Metro#Technology They use a VAL kind of train: https://en.wikipedia.org/wiki/Véhicule_Automatique_Léger https://en.wikipedia.org/wiki/Véhicule_Automatique_Léger The excavation diameter for the tunnel is (one track) typically 5.3 m and 7.7 m (two tracks), the net diameter inside is smaller of course, depending on the thickness of the concrete lining, usually 4.5 and 6.7 m, respectively. Cars are 2.06 m wide and of course tall enough for a standing person (+undercarriage) 3.25 m, there is not much you can do to further reduce their size.