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But Chris - who spotted the animal near Faslane naval base where he works - soon realised it was something more chilling.
He said: "It wasn't moving the way I expected a dog to. It was then I realised what I was seeing was a big cat.
"I ran to grab my mobile phone for a picture. I stood on the rail bridge and got a still photo and a couple of minutes of footage of the animal moving up the railway line. It was remarkable."
In the clip, taken close to a pal's house in Helensburgh, Dunbartonshire, the cat moves up the line and at one point seems to balance and walk along a section of rail.
Chris, originally from Huddersfield, added: "I've heard stories about creatures like this moving about the countryside but never really believed them before.
"Looking at the video, I don't think there's any doubt it's a big cat."
Faslane pest controller John Belshaw has been contacted about big cat sightings before.
He added: "I have had a look at Chris's footage and have to say I do not believe it is a domestic cat or dog.
"At one point it seems to walk on the railway line and a dog simply wouldn't have had the balance to do this. You can tell it is much larger than a house cat."
Shaun Stevens, of research group Big Cats In Britain, added: "Knowing the width of the tracks in Chris's video, the animal is clearly in excess of 4ft.
"I think this could be one of the best pieces of footage of a big cat in the UK."
Several sightings have been reported in the area, and BCIB get around 30 in nearby Argyll each year.
Just a few miles away from Faslane, sightings of the tan-coloured Coulport Cougar began in 2004.
A large black cat has been spotted at Garelochhead Training Camp, two miles away.
And last week we told how a puma was being hunted in Ayrshire after it savaged a horse in Coylton.