The realisation that the events of July 7 were not an isolated conspiracy has changed the way that we travel on the city’s public transport system, probably for ever.

On the face of it, yesterday morning was like any other as I started out from High Barnet shortly after 8am. Commuters from this North London suburb grabbed cappuccinos from the small kiosk at the station and rushed to catch the Northern Line southwards. But a member of the station staff said that there were fewer people than usual. “The car park is half empty. It’s usually pretty full by now. After what happened yesterday, people here decided not to go to work today or are going by car.”

Those boarding the train expressed the same stoicism that characterised the reaction to the bombings a fortnight ago, but it was tinged with unease. Angela Leonard, a new mother, said that her husband had not been keen on her taking the Tube but there was no other feasible way of reaching her office, and in any case, “I just want to carry on with what I have always done”.

As the train trundled off past leafy back gardens passengers had their noses in Harry Potter and Su Doku books and women whipped out compacts to put their faces on as normal.

But the headlines on the fronts of the papers told a different story. And so too, if you watched for a couple of minutes, did the behaviour of the passengers. They frequently lifted their heads to scan the carriage. At each station, those entering and leaving were clocked by dozens of pairs of swivelling eyeballs.

However nonchalant we all tried to be, it was not subtle. The first thing that everyone looked for was the type of bag a new arrival was carrying. Anything bulky, anything that looked like a rucksack, warranted closer observation. And there was no question that passengers were profiling their fellow commuters in another way.

Yes, they were looking at the colour of their skin. A young Asian man, smartly dressed in a suit, got on with a bulky black rucksack. I cannot pretend that I did not give him a second look. No alarm bells rang, but I could see other people stealing glances too.

And so could the poor chap, who was probably looking forward to a nice weekend away somewhere. He fidgeted a little. Who could blame him, the way his fellow citizens were behaving? But the more he fidgeted, the more other passengers twitched. He got off after a couple of stops. The man sitting opposite raised his eyebrows at me. “You would think today he might have done without the rucksack,” he said.

Bizarrely for a rush-hour Northern Line train there were plenty of seats available, even at Tottenham Court Road, and then at Charing Cross we came to a halt. In more innocent times this would have been tedious, but the announcement that the line was being suspended because of a suspicious package at Kennington was a cause for more than irritation.

Giving up on public transport, I took a taxi. The driver was Muslim. I told him about the racial profiling I had detected and that I felt guilty about it.

His silence felt like a reproach, then after a minute he condemned the terrorists in the most forceful way I have heard from anyone. “If they catch them, they should torture them,” he said. “And if they won’t do it, they should give them to a country that will.”

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