One of those awful moments of epiphany, but in a bad way - the moment the rose-tinted specs were suddenly ripped off your nose. Today, on the District Line, Earls Court Station, platform 2-3, Eastbound, at about 5.20 pm.
Two trains arrive at the same time, there's that interval of uncertainty where people are trying to decide which one to get onto as both go East. But one will leave a minute before the other.
I choose the emptier train, the one going, I think, to Upminster. I am followed into the carriage by three jovial middle-aged men, casually dressed, chatting in what seemed a good natured way about the vagaries of the tube system.
As we wait for the doors to close, the impression that these are three nice gentlemen is reinforced as they joke with people jumping onto the train. They look like three good dads on a day out, late 50s maybe, all a bit stocky, checked shirts and chinos and trainers, nothing unusual there, they're just a little bit noisy.
Yes, they are talking rather more determinedly, more pointedly, than you'd expect at this commuter time of day, as though they'd been drinking a little. But still good natured, cheery.
The doors close and then one of them says something in a way which - suddenly, decisively - transforms the atmosphere in that half-full carriage, and turns the solid contents of my bowels into water.
He says something so shocking I cannot remember the exact words, and he says it in such a way - in such a different, aggressive way to the way he had been speaking - that you know it is his intention that everyone in that carriage should hear him and understand his meaning.
He says, "Well, well, this carriage is almost all white."
As he says this I think, what does he mean? And I actually look up at the ceiling of the tube carriage and see that it is off-white but quite clean and think for about a millisecond that this must be what he means.
And then of course I realise that that is not what he means because his two mates are grinning and swivelling their heads around looking at the people in the carriage, and nodding, and I think I hear one of them say "Yeah not many scum here" or something to that effect, and looking around too I realise that indeed what he meant by almost all white was that there were no black people in the carriage.
But there were a couple of women who were already sensing something extreme and unpleasant in the air.
The train drags itself through Gloucester Road, and now every yard of its progress seems to take forever, as though their words had not only polluted the air, but made it heavier. I try not to listen.
I continue to read my Evening Standard, I fold over the page, and the man who spoke earlier looks very hard at the page I folded.
"I can't believe it", he said. "Look at that. That fucking scum. That fucking shit, and those other shits those scum, look at that, they're at the 02. At the O2. Can you believe it?"
As he says this he's trying quite hard to catch my eyes but I bury my face deeper into the newspaper. Already I know what he's referring to. The page I have folded away from me to face him has a quarter page ad for a benefit conceert at the O2 for the Stephen Lawrence foundation, with a stylised portrait of Lawrence superimposed onto a Union flag.
I feel that at exactly that moment everyone else in the carriage also realised we were witnessing a display of hard-core racist provocation.
Perhaps they feel safer in the "almost all white" carriage, but I don't think that is it.
It's clear they are saying these things in this new and completely frightening tone with the specific purpose of challenging anyone there to take them on. To challenge this utterly astonishing racism.
And to make it so that if you do not challenge them you are actively condoning what they are saying.
And no-one does challenge them. So they continue, and then one of them says, "Well perhaps we should quieten down becuase it looks like we're upsetting a few people here."
But still no-one says anything, there's that sort of stupid stretched skin smile on a few faces, heads down for most.
So this is my greatest shame for today. I imagined saying out loud, something, anything, just to show that I'd heard what they were saying and that it was not OK to say those things like that. I should have said something like "fuck you, you racist pigs, get back to your styes in the shit-filled fields of wherever you emerged from into this sad world, so much sadder thanks to your presence within it."
But I didn't say a word. I din't even give them a filthy look.
For pity's sake - in 2013 in London - why?
Pure cowardice. When I looked at them again I realised it. These were the same men - older, plumper, but the essentially the same men - as the thugs I'd last seen in the wallowing down the King's Road chanting racist obscenities as they went to their football match.
Here, they were committing a crime and we were all accessories. These men were testing the water, and they must have felt very satisfied at the result, even though they would really have loved a fight, the chance to put the boot in.
By not saying anything, I have strengthened racism in Britain in 2013, and I am ashamed of myself.