Becoming a train driver was not a boyhood dream for Craig Puffett. His grandfather and uncles all worked in the industry, and he had plenty of fun on the local miniature railway as a child, but Puffett's ambition to join the railways developed later in life.

"At school, careers guidance was geared towards a professional career, and I started out as a trainee quantity surveyor," Puffett, 42, says. "But the recession meant I didn't get an enormous amount of experience. After a while I went to work for an internal auditing and accountancy firm, but I found working in an office environment too stuffy and political for me. It wasn't my cup of tea."

He stuck out office life for several years before deciding he needed a change. He came across an advert for a train driving position and applied, but wasn't successful. He kept on searching and eventually took a job as a chauffeur. "It was a long way from office work and I really enjoyed it. I did that for five years, but in the end the hours and pay were unpredictable, and as my wife and I were starting a family I decided I needed something more reliable."

Train driving was still in the back of his mind and before long he saw an advert for what was then Silverlink Metro, now London Overground Rail Operations Ltd (Lorol), inviting applications for trainee train drivers working from London Euston and Watford, near the family's home. "I thought: 'I've got another chance'," Puffett says. It was months before a reply came back, asking him to spend a day at an assessment centre to see if he was cut out for the job.

The company does not have any specific requirements when it comes to qualifications or work experience for applicants, but the recruitment process is very tough. Puffett underwent a number of assessments, including a psychometric test, listening and reading comprehension, mechanical comprehension, and a structured interview. "There were six of us and they whittled it down during the day. If you didn't pass a test you were told as much and left. Two out of six of us made it through. It was nerve-racking."

Puffett's preparation for the tests, which included weeks playing 'Bop It', a children's reflex game, and Tetris, to get his reaction speeds up, paid off. After another interview with the driver manager and HR manager he was finally offered a place on the training programme – which was every bit as rigorous as the selection process.

The programme is modular and trainees have to pass each stage before they can move on to the next. "The first nine or 10 weeks were in the classroom, learning about trackside rules and regulations. There was four weeks' traction training on trains we would be driving, and training on how trains work and the sorts of faults that might occur."

"You have to do 200 hours with an instructor, with a third taking place in the dark," Puffett explains. "Then there is a week-long assessment based on everything you have learned." It should take nine months to complete, but for Puffett a shortage of instructors made it longer.

Only once enough hours have been clocked up can trainee drivers finally go solo. "I was nervous on the day," he says. "You keep your fingers crossed that nothing will go wrong – even though you have had the training you still don't want it to happen. It was strange not to have anyone there, but it was fine."

The prospect of potential problems keeps drivers on their toes, but Puffett says it is not normally a stressful job. "It is usually incident-free, although I did have a run in with a dozy pigeon once. It fell on the train right where the electric cable is and blew the power out. I had to do the walk of shame down the station platform to explain the delay to the passengers."

Puffett says that to do the job you need to be self-reliant, as a lot of the day is spent alone. "And you have to be happy to follow procedures to the letter. You can't improvise," he says.

The shift work would not suit everyone –he starts at 4.20am to drive the first train of the day, and late shifts finish at 1am, and a lot of weekends are spent at work. But his shift pattern means a four-day week, and a week off work at the end of each four-week roster.

According to Lorol, while some trainees are college leavers, many train drivers apply for the job having already followed a career in a different line of work, so Puffett's career change is not unusual.

"The pay and benefits are pretty good and the shift work suits me," Puffett says. Trainee drivers start with a salary of £24,024 rising to £39,372 after the training period, while experienced drivers earn £50,451. There is a final salary pension and rail travel for free or at a discount rate.

"This is the longest I have ever stuck with a career and I can see myself staying," Puffett says. "It's one of the few areas left where you can really have a job for life."