As you increase speed, you need to take turns at larger and larger curves to reduce g-forces on passengers, and the curve radius increases exponentially vs speed. The bigger the curve, the more land you need to buy, and depending on which country you're in property rights extend above and below the land so the cost is unavoidable. (If you can avoid it by going underground or overground, the cost of tunneling and bridges is expensive.) It becomes increasingly costly to increase speed, and on top of that you get diminishing returns with travel time (speeding up from 80 to 120MPH is worth a lot more than going from 200MPH to 240MPH). And above a certain point you spend too much time accelerating and decelerating from top speed, and not enough time actually at the top speed to make it all worthwhile.

Regardless of motive technology, curve-radius will always be a relevant problem, because one of the major advantages of the train is that you can walk around and about. We could design trains with sharper corners and harder acceleration/deceleration, but no one wants to be strapped into a vomit comet for three hours.