Compact Tunnel Boring Machines Are Major Players in North America

Less wide tracks = less comfort and less stability, as the trains will tilt left and right more rapidly.

Because passengers still would need to walk along the train, and that gangway is sized to accommodate humans with their luggage, you also would have fewer seats per square meter, or require way more doors per carriage.

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/Toulouse_Metro https://en.wikipedia.org/wiki/Toulouse_Metro#Technology They use a VAL kind of train: 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.