The problem is not that they cannot imagine the concept of adding new lines, it's just that the costs, both monetarily and politically for doing so, are enormous in the Western world. The eminent domain, permitting, and labor issues around these things paralyzes the development of cities, especially in the United States.

If you want to see ambition in infrastructure, check out China. Being a technocracy bent on having world-class infrastructure, combined with the ability to remove people from their homes for the greater good with little repercussion, they've built some of the most thorough infrastructural improvements ever seen on the planet. For instance, the first line of the Shanghai Metro was opened in 1993; today, it is the longest metro in the world, with 14 lines. And this rapid growth can be seen not just in the megacities like Shanghai, but in countless smaller ones throughout the country—not to mention their incredible construction of their thorough long distance high speed rail network. And on top of that, since their system is so modern, the trains are a hell of a lot safer and more reliable than any piece of infrastructure in the USA.

I think they've made mistakes with how car-oriented the streetscapes are, the generic architectural styles throughout that country, overbuilding before demand arises, and a lack of mixed-use neighborhood zoning, amongst other things; but when it comes to imagining an integrated, efficient commuter rail network, they've killed it.