> it might become more interesting to dig long tunnels under cities for these trains rather than increasing the train's maximum speed. South Korea built >50 km

> it might become more interesting to dig long tunnels under cities for these trains rather than increasing the train's maximum speed. South Korea built >50 km tunnel out of Seoul for this reason. It opened in 2016. https://en.wikipedia.org/wiki/Yulhyeon_Tunnel https://en.wikipedia.org/wiki/Yulhyeon_Tunnel