'I knew you weren't proper builders,' I said to the two men who had been causing minor discomfort to self with tools which hung from their belt loops every time they moved.

They'd squashed themselves into the non-existent space by the train doors moments after I'd somehow got myself into the full-to-bursting carriage, and moments later revealed, as I had suspected, they were not proper builders but plain clothes ticket inspectors.

"No," said one. "We're inspectors. But we're working undercover to try to unearth fare dodgers."

"And smokers," I said, since they had just dramatically challenged and evicted someone who had dared to light up in the presence of what he obviously thought were two 40-a-day builder types.

"Well, not specifically," said the other. "But I could see he was about to set fire to your hair. So it only seemed fair to challenge him on that too."

"Anyway," said the other who had shown the smoker off the train at the next station. "He didn't have a ticket either. If they break one rule, they usually stretch the others as well."

"But," they both said at once, "How did you know we weren't real builders?"

"Well, mostly because you were on the train in the first place," I replied. "I mean, builders have vans and lots of tools which they put in their vans. Not just a couple of hammers hanging from their belts.

"Oh, yes, and you're supposed to do the belt up halfway down your buttocks, not round your waist - that was a bit of a give-away."

"Yeah, well, we'll have to rethink the outfit, I suppose."

"Why were you dressed as builders in the first place?" I wondered.

"Well, this is a plain clothes operation. So we can catch people who haven't got tickets without them seeing us coming.

"We're encouraged to dress up as different professions, you see."

I thought I did. At least I imagined a group of ticket inspectors arriving for work in their blue and yellows and being led to the dressing-up room in the middle of which was a huge blue and yellow box stuffed with dressing-up clothes.

"What shall we be today then?" they exclaimed with delight, pulling out doctors' bags and firefighters' uniforms from the box, before discovering a hard hat and a pair of jeans which looked a bit too tight and deciding that today they'd go for builders.

But it probably wasn't really like that. "So, what will you be tomorrow?" I asked.

"Well, we haven't really decided yet," said one. "But you're right about builders not really travelling by train. It'll have to be something a bit more commuterish."

The following day I was getting increasingly irritated by two phones in the carriage which kept ringing. The owner of one was sitting in the seat behind me.

So, although I could not see him, I could hear him answering and saying things like "Yes, yes. Sell. Have you been on to Tokyo yet. Okay get back to me."

When I stood up to get my coat, I saw one of the 'builders' wearing an ill-fitting pinstriped suit and yelling into a mobile phone. He stopped when he saw me.

"What do you think of the disguise today?" he beamed. "We're stockbrokers," he smiled, indicating the other former 'builder', who was sitting at the back of the carriage in a suit on the phone, discussing futures markets.

"And are you targeting fare dodgers today?" I asked, "or mobile phone users?"