Why Trains Haul Locomotives That Are Completely Switched Off

Every few days, somewhere along the freight corridors of North America, a train rolls past carrying something unusual. Tucked between the lead locomotives and the freight cars sits another diesel engine, over two hundred tons of steel and machinery, painted in the same livery as the working units up front. But its exhaust stacks are cold and its prime mover is completely silent. This is not a mistake or a breakdown that caught the crew off guard. So why would a railroad deliberately haul a multi-million dollar locomotive as if it were just another piece of cargo?