Dandong, the Chinese border city that faces North Korea across the Yalu River, is a barometer of ties between Beijing and Pyongyang. While lockdowns during the coronavirus pandemic led North Korea to sink into even greater isolation than usual, the resumption of cross-border rail links in recent months has triggered a pickup in activity, with freight volumes rising and local travel agencies reporting a steady stream of inquiries. In March, the first passenger trains crossed the border since...