A long distance train benefits from very high top speeds, because it spends a lot of time at those top speeds. Conversely, increasing the high speed of a subway train is subject to the law of diminishing returns, since the train has to comfortably accelerate and decelerate within the span of half a mile. With subways, the main concern is that every single train operates in pretty much the exact same manner WRT acceleration, deceleration, and speed, since the headways on subways can be measured in minutes, or even seconds.