"I have bided my time to name a photograph after a Johnny Cash song. It has been an exercise in employing patience because playing this card required not just a strong image, but also an emphatic connection with the song in question.

"But now we have our moment as this photograph can surely only be called 'I Walk the Line'. It was taken on a cold January morning on the train tracks north of Rockwell on the Durango to Silverton train line in Colorado.

"The cold makes the photograph. It was around 20 degrees below zero, allowing the bison´s breath to become an important prop in the image. In most cases, any feature that defies gravity is a bonus for a picture as it allows a story to gain breadth and depth.

"Over the years, we have built strong friendships with the employees of this historic railroad and, of course, its owners. The relationship of mutual trust that has organically evolved has made our assignments there progressively easier - we could not have pulled this idea off on our first date." - David Yarrow