Written by Jethro Tull frontman Ian Anderson, the locomotive in this song is running out of control, and is a metaphor for societal problems. Anderson gave a detailed explanation of the song in our 2013 interview
, where he said: "When I wrote it, I wasn't deliberately setting out to write a piece of music on a particular subject. But it evolved during the writing process into being not terribly specific, but about the issues of overcrowding - the rather claustrophobic feel of a lot of people in a limited space. And the idea of the incessant unstoppable locomotive being metaphor for seemingly the unstoppable population expansion on planet Earth.
When I look at it today, it does, for me, become very crystallized in being a song about unmanageable population expansion. It's something that concerns me even more today than it did back when I wrote it, when the population of planet Earth was only about two thirds of what it is today. So in my lifetime alone, we've seen an enormous increase in population, and an enormous increase in the degree to which we devour our limited resources. So the idea of population planning and management is something that I think we ought to be thinking about a lot more than we do. Does that mean I think we should sterilize everybody after the age of 30? No, of course not. The size of the family you want to have is going to be your choice. But, you should make that choice knowingly, wisely, and responsibly."