Even turns in the vertical direction are a problem, for uncomfortably pushing you into your seat, or lifting the train off the rails. Which results in more brid

Even turns in the vertical direction are a problem, for uncomfortably pushing you into your seat, or lifting the train off the rails. Which results in more bridges & tunnels than would otherwise be needed. Unlike slow trains, there is plenty of power for going up hills, but you can't switch to going down the other side. While ordinary horizontal curves are cambered, IIRC the resulting train-vertical acceleration is still one of the limiting factors.