Aides panic as William and Kate make spur of the moment decision to edge along railway tracks next to steam train (photos)

One foot wrong and they could have been sent tumbling into the freezing water.

But the Duke and Duchess of Cambridge looked fearless as they balanced precariously on the edge of a railway line, looking down at a river several feet below.

After touring a steam train, the couple climbed down from the cab while it was stationed on a bridge on the White Pass and Yukon Route railway, at Carcross, Canada, the Daily Mail reports.

Kate was left facing outwards towards a drop but quickly regained her composure and sidestepped towards solid ground, her husband following close behind.

The royals made the spur-of-the-moment decision after being told the Queen and Prince Philip had travelled on the train when they visited the region in 1959.

But while the Cambridges' appeared to revel in the adventure, it left their aides momentarily panic-stricken.