OH dear. Of all the things to get wrong.

Last week's Guidelines spent much of its 1,000 words or so extolling the virtues of trainspotting.

We went back to the hot summer of 1970 and hours spent on the concrete wall at Sands Road watching the diesels come and go.

My father-in-law went back further, recalling the roaring steam locomotives thundering down the express lines through Swindon during his boyhood.

So far, so good.

But it all went wrong when we chose a picture from our archives to illustrate the article.

I'm not even going to describe it this week. I'm not going to say which loco we thought it was, nor which loco it actually turned out to be, because I don't want to risk getting it wrong again.

Suffice to say, the engine in the picture did not match up with the name in the caption.

For a few moments after the paper hit the streets last Tuesday, all was quiet.

There was little to disturb the contented slumber of the reporters here at Harmsworth House, nothing but the ticking of the clock and the distant sound of photographers cleaning their fingernails with war surplus bayonets and pulling the legs off spiders to stir their little tousled heads from deep sleeps, dreaming of embargoes and shorthand outlines.

But then the phone began to ring.

And it hardly stopped all day.

Who would have guessed that so many people would spot our error?

Some were quite irritated, but most were extremely pleasant.

A photographer from Marldon went so far as to send me one of his own excellent pictures, showing the newly-built loco Tornado on a damp day at Goodrington.

One chap rang and told me so earnestly that there was a crucial and very serious error in the Herald and Express that I thought we must have perpetrated a terrible libel.

My old friend Dave rang to point out the error of my ways, but did it so nicely that I actually felt better after the call than I had before it.

He told me how he used to go spotting in his younger days in London, choosing to spot from the platform at Haringey rather than Kings Cross, because it was free to go on the platform at Haringey while it cost a whole penny to do so at Kings Cross.

A lady from Paignton wrote me a lovely letter about the girls who used to train-spot back in her childhood, watching the Golden Arrow roar across their nearest level crossing at Tonbridge in Kent on a Tuesday, with its carriages filled with posh tables and lights.

I looked it up. The Golden Arrow was a boat train that ran from London to Dover, with Pullman carriages for the rich and famous.

I had a phone call from a former colleague a lifelong rail enthusiast who said I had completely upset his digestion over a tincture at his club, and should correct my error at the first possible opportunity.

Even my father-in-law himself took me to task for the picture.

Who knew that so many people would not only spot the mistake, but also go to the trouble of pointing it out?

And the thing was, they all did it so nicely, which kind of proved the point I was trying to make last week.

Spotters, enthusiasts, call them what you will. They are all united by the fact that jotting down a number is just a way of recalling a bigger and more vivid memory, and one they are all just bursting to share.

So I would dearly, dearly love to say that our picture/caption howler was a deliberate mistake designed to set the steam enthusiasts a puzzle.

But it wasn't. It was just a mistake, pure and simple, and I'm very, very sorry.