I used to know a man who left his home near Farnborough in Surrey at about 6am every morning so he could avoid a traffic jam on the M25 on his way to his work, somewhere near Watford. He'd then stay at his desk until 8pm to avoid the return jams. He was putting in about 11 hours each day and I doubt he was paid for more than eight-and-a-half of those. Either way, he hardly ever saw his family.

Another man I knew lived in Doncaster but worked in London. His commute started at 6am every morning so he could catch the 6.30am train. Twelve hours later, he'd board a train home which, if he was lucky, would get him home by 9pm. That's roughly three-and-a-half hours he was spending on the train, five days a week.

How on earth did he spend the time I wondered. "Oh that's easy," he said. "I take my laptop. It's amazing how much work I get done." Did he get time off in lieu for these extra hours, or did his employer allow him to flex his hours in return? Of course not - he was simply gifting the company with an additional third of work on top of his paid working time. And he was setting impossibly high expectations for his successor.

It was the choice of these men to live where they did and work far away. They assumed they got paid for the hours they spent in the office and that travelling time was their own responsibility.

But a new study from Sheffield University's Institute of Work Psychology has shone light on our attitude towards working while commuting. The researchers, Carolyn Axtell and Donald Hislop, concluded many commuters were using trains as mobile offices to improve their work-life balance by releasing the pressure they would otherwise feel if they took their work home.

Some of the travellers said they found working in transit "liberating", and they liked the neutral workspace because they were able to choose how and when they worked.

On the downside, the researchers highlighed an increased blurring of the boundaries between home and work thanks to Blackberries, mobiles and wireless-free internet. And some people were using their 'mobile office' irresponsibly by discussing redundancies over the phone, for example, or giving out the office security codes.

For now, the typical working commuter is likely to be male, working in management, sales or engineering. It remains to be seen how long it will take employers to recognise this as a valid part of the working day, and to start the clock from the moment staff pass the ticket barrier, rather than the reception desk.

Steve Crabb is editor of People Management