A funny thing happened to me at the railway station. I was overcome by the desire to buy a book about railways. In fact I might as well come clean. It was a book about diesel locomotives.

But before you get the wrong idea, let me explain. My interest in these matters is entirely romantic. I like big machines. If they are old and dirty and emit unhealthy fumes, then so much the better. The combination of noise, energy and danger still thrills the little boy in me, and I marvel to think that, when I really was a boy, these growling giants were new, and have grown old with me.

But the book I bought - in haste, because I had a train to catch - well, it wasn’t like that at all. Page after page dealt with the minutiae of livery variations, the positioning of marker lights and the vagaries of numbering.

“Here is a rare sighting of No. 47345, heading the 11.10 Waterloo to Exeter (St David’s) service, passing the former Nine Elms sheds on 14th July 1985 (note the now standard headlamp arrangement).”

“And here, on Sunday 2nd May 1965, is No. D1733 (later No. 47614) in an xperimental blue livery which was the precursor of British Rail’s slightly darker ‘Rail Blue’ .” (the photograph, I might add, is in black and white.) All this smacks of fetishism. By which I mean that the true object of desire (okay, so I fancy diesels) has been displaced by a lot of peripheral nonsense about logos, paintwork and route indicator panels. And running through it all, like a night mail train on moonlit metals, is the compulsion to classify and collect.

This, it seems, is what humans enjoy most. If we’re not stuck at the stage of spotting locos, collecting stamps or memorising football results, then we’re taping and labelling TV serials and climbing every Lakeland fell with a summit in excess of 1,000 feet.

Presumably this tendency had some function early in our evolution, although I can’t think what. The urge seems stronger in males, though. Which is interesting when you consider that boys are about four times as likely as girls to have autistic spectrum disorders. It’s hard to give precise figures, of course. But you can bet your life that some guy’s working on it right now.