>5-minute walkable neighborhoods. The proximity to services of cities combined with the roominess of suburbs. At least in theory it's the perfect place to live.

The problem with 175 km of continuous "subway" is that you probably don't want to walk more than 2.5 km to the train (if that, it's Arabian hot) so you have 35 stops (5 km apart) along the line at a minimum — with a blistering acceleration of g = 10 m/s^2 you're stuck at an average velocity of 18 km/h, so it's eleven hours of travel end-to-end assuming two minutes at each stop. Those are optimistic assumptions. You can mitigate it slightly by having express trains skip stops, but now it gets complicated, etc. To be anywhere near reasonable you probably want about five actual transit lines on the single "line".

A much less crazy plan would have 8-16 compact neighborhoods at various spacings along the line, where you can get the most out of the directional freedom of walking, and the train can go much faster without having so many stops.