The Great Northern in Coulee City??

How does a Great Northern train become wrecked in Coulee City, which is clearly Northern Pacific country?
Back in 1910 (not 1909), right after the opening of the line between Coulee City and Adrian, flooding on the Great Northern at Wilson Creek caused the GN to reroute the Oriental Limited over this NP branch out of Spokane, to be lined back to the GN at Adrian. Someone lined a switch at near Coulee City wrong and the GN train went off the end of track on the west side of town, instead of going south to Adrian. They had time to slow down some, but the engine did fall down the end of track.