The tube station clang

It's not very often that I get excited when I hear a certain noise on my way to work each day. But there's a clang, a heavy clang, in the tube station I leave from every once in a while that fills my heart with glee. It's a heavy metal clang which signifies something's happening with the track or the points or whatever it is in the station in the tunnel out of view, and it can mean only one thing. An empty train is coming and I will get a seat.