Who's There? is a magazine story.
Peter Sam is finding it hard as he travels through the fog. But, when he blows his whistle, he hears a reply which puzzles him. To make things more confusing, he sees the out-line of another steam engine in the fog. Later, Peter Sam tells the Fat Controller what he saw. When the sun burns off the fog, Peter Sam goes back along the previous route to discover that the out-line of another steam engine is a wooden panel shaped like himself with his livery. Also, he realised why he heard an another whistle - it was an echo of his own whistle!