Eminent domain, not imminent. There's no connection to time, but the state has the absolute power to seize property; in many countries (including the US) this is limited by constitution and regulation to specific purposes and to require just compensation.

Another factor in the US is that our railroads are generally freight oriented. Routing to have three ground based networks is challenging (road, freight rail, passenger high speed rail), especially in urban areas where right of way is expensive and also over mountain passes. A lot of urban freight rail lines are currently unused; some of them become greenway rail trails, others sit unused and may be at risk of encroachment, but often the alignment isn't useful for high speed rail anyway.

For the record basically all of Shinkansen in Japan is new standard gauge rail, as "normal" trains in Japan are narrow gauge. And they still manage.

In some places you might have shinkansen, multiple private narrow gauge networks, metro and regular roads going through the same piece of land.