THE BRIDGE AT TSAVO, 1898   "Descent approaching now," the guide had said perched familiar to the hunching train. Steam obscured valley and track, dead   tangles glimpsed forming patterns like pain housed in the far-off smoke where toil channels extension of an awakened skein   to bridge two worlds. Hours it has taken to join the work camp, passing walls of thorn in which the animals blink and turn, avoid   confronting the mys