If you've ever sat in a train carriage and felt a steady thud coming up through the floor, a soft clunk, clunk, clunk in time with the wheels, you were feeling something real. That train had a flat tire. And here's the part that catches people off guard. A train wheel actually has a tire. It's a separate steel ring, wrapped around the wheel and held on by nothing but its own grip. So how does a solid steel wheel go flat? And why did railway engineers spend more than a century learning to fear that quiet little clunk?