There were strict rules for commuting, laws as immutable as gravity.

It was forbidden to show emotion. A commuter must be neither happy nor sad. The only acceptable expression was one of mild boredom, even if tomorrow you were off to sunny climes with the partner of your choice, unlimited booze and a promise of more hot sex than anyone could reasonably hope for.

No eye contact was allowed. Never mind that the carriage was so crowded the umbrella of the gentleman in front was in contact with you more intimately than anything had been for too long. You might be facing the man or woman of your dreams; if by chance your eyes did meet across the crowded carriage, you should look away hurriedly.

Under no circumstances should you strike up a conversation, even if the person next to you was your spouse of twenty-five years. Anything that might be said should have been communicated before the journey started, or could wait for the privacy of the office.

There were exceptions. Delays of twenty minutes or more could be greeted with mild irritation, or even a tut or two. Dirty looks could be directed at tourists with backpacks or passionate couples sucking noisily at each other's tongue. In dire emergencies, intimate acquaintances might murmur to each other in hushed tones.

Mobile telephones were a wild card. You could talk as loudly as you liked, about the most intimate of practices or the most sensitive of financial details, provided you had a mobile phone pressed tight to your ear.

But these were the exceptions, and if you were in any doubt, the safest course of action was to maintain a bored indifference.

Sometimes, though, events came about that overturned the natural order of things, and the normal laws no longer applied.

This morning was a case in point. The train was stationary, half a mile short of the station, and had been beached hopelessly there for the last twenty minutes. Delays outside the busy terminals were par for the course, and the seasoned traveller did not even notice them. But twenty minutes was exceptional, even for this busiest of times.

In the standing area by the doors, the four of us waited. The mild irritation rule had been invoked by the large lady in the floral dress. The city gent had forcibly refolded his Financial Times, his frustration exaggerated by the knowledge that even these extra twenty minutes had not enabled him to answer twenty-seven down, Die of cold (3-4). The young secretary stabbed at the buttons on her MP3 player, none of the tracks compensating for the delay. I lounged against doors, eyes half closed, in the vain hope that if I looked as though I were asleep, I might end the journey more refreshed than when I started.

The public address system crackled and hissed, the familiar precursor of some specious excuse as to why we were going to be delayed yet again. Earphones were removed and papers lowered as we waited for the announcement.

"Good morning, ladies and gentlemen. I am sorry for the delay this morning. We're at present waiting for a green light, but there is heavy congestion at the moment, due to a customer committing suicide on the line at London Bridge. Unfortunately, we are going to have to wait until the line is cleared before we can move on. As soon as I have any more information I'll let you know. Once again, apologies for any inconvenience this might cause you."

"Oh my God! A suicide?" said the young secretary, eyes wide.

The floral lady shook her head. "What goes through their mind? Don't they have any family? It's so selfish."

The city gent shrugged. "That's symptomatic of today's society. There's no cohesion, no sense of belonging. Everyone thinks the world starts and stops with them."

"Still, poor soul," sympathised the secretary.

We all paused, meditating on the loneliness of the city, the brevity of life and our own mortality.

Then we did it. As surreptitiously as we could, because deep down we were ashamed, yet we were unable to suppress it. As one, we all turned our wrists and looked at our watches.