Narrower tunnels?
There is a certain problem with high-speed travel in a tunnel. A vehicle has to push the air out of the way. That is why high-speed trainsets have very aerodynamic ends. If there is not much space between the vehicle and the tunnel, then it will be hard to squeeze the pushed-aside air through it.
Consider the
Gotthard Base Tunnel - Wikipedia, designed for 250 km/h travel. It uses twin single-track bores, and each one has a diameter of 8.83 - 9.58 m. Its tunnel clearance is 5.20 m (rail top to overhead conductor). From
Loading gauge - Wikipedia, UIC GC loading gauge is 3.15 m width and 4.65 m height. This is plenty of space for pushed-aside air.
Urban-transit systems have narrower tunnels. For instance, BART's Transbay Tube has a diameter of 17 ft / 5.2 m. For equal width and height, this means an envelope with a width and height of 3.7 m, something that seems roughly correct. Measuring a picture in
HSLRST / Archimedes Bridge Studies gives us:
Height of tunnel floor: 0.87 m
Width of BART train: 3.42 m
Height of bottom of BART train: 0.38 m
Height of top of BART train: 3.42 m
Max speed: 80 mph / 129 km/h
Cross-section fractions:
Gotthard Base Tunnel, GB C: 0.24
Transbay Tube, BART: 0.54
Some London Underground rolling stock seems like it closely fits its tunnel roofs, and it likely has a greater cross-section fraction.