It may have sent some critics hurtling round the bend, but the BBC’s Glasgow to London train-set thriller Nightsleeper is proving a runaway success internationally.

Producers Fremantle have sold the six-part series to 146 territories worldwide, including Australia, Denmark, France, the Netherlands, Portugal, Iceland, Japan, and New Zealand.

The global sales follow ratings success at home. The first episode of the BBC One and BBC iPlayer series attracted 8.5 million viewers, making Nightsleeper one of the UK's most-watched dramas of the year.

Written by BAFTA award-winning writer Nick Leather (The Control Room), Nightsleeper is set aboard a Glasgow Central to London Euston service that has been hijacked, or “hack-jacked”, by shadowy forces.

With the train under the hackers’ control and heading towards London at full speed, it falls to a group of passengers, led by an ex-policeman (played by Joe Cole) and a transport department civil servant (Alexandra Roach), to save the day.

Starring a host of Scottish actors including James Cosmo, Sharon Small, Leah MacRae and Sharon Rooney, Nightsleeper had a mixed reception from domestic critics. The Herald gave it two stars, describing it as “six-hours of bum-numbing nonsense that was about as enjoyable as the night bus to London”, though it was praised for making Glasgow look great.

The Guardian, also two stars, called it “fantastically dreadful”. The Telegraph awarded three stars, calling Nightsleeper "the TV equivalent of rubber-necking: you know you should look away but you really can't", while the Times thought it “very schlocky” with “implausibilities from the opening frame”.

The Daily Express was on the side of fans, dubbing Nightsleeper a “twisted, high-octane hit”. One viewer compared it to Line of Duty and Bodyguard.

The production’s accuracy was much discussed online. One early debate raged around why the authorities did not simply bring the train to a halt by cutting the overhead power. Answer: there was a diesel back-up.

Internationally, audiences and critics alike have embraced the show. In Australia, the Sydney Morning Herald listed Nightsleeper among the best new shows to stream in October, and in France the first episode was watched by 3.25 million viewers.

There has been no word yet of a second series. Radio Times reckoned the earliest one could arrive is late 2025 or 2026. If the production does not want to repeat the Glasgow to London plot, Caledonian Sleeper trains also run between London and Aberdeen, Inverness and Fort William.

Jens Richter, CEO commercial and international at Fremantle said: “We’re thrilled to be partnering with such an exciting mix of esteemed global broadcasters to continue introducing Nightsleeper to audiences around the world. The series delivers pulse-pounding action and unpredictable twists, whilst featuring the exceptional talents of Joe Cole, Alexandra Roach, and an outstanding ensemble cast.’’ Kate Harwood, Executive Producer, said: “Nightsleeper was a demanding production, but its success has been immensely satisfying and fun to experience.”

The series is directed by Jamie Magnus Stone (Dr Who) and John Hayes (Dublin Murders), and was shot mainly in Glasgow.

Sharon Small, who plays a Tory transport minister, is based in London but was born in Glasgow. Making the thriller was her chance to visit family. In an interview with The Herald’s Susan Swarbrick, she said: “I spent time catching up with my nephew and new wee niece. That was lovely. I love going back [to Scotland] and it seems it is only work that takes me back now. I really enjoyed hanging around Glasgow.”

Smith is firmly with the fans on Nightsleeper. “Honestly, I thought the premise was brilliant,” he said. “The writing was so well crafted. It’s funny. The characters are all really different. It was ambitious and I thought, ‘This is great fun.’ And who doesn’t want to be in a heist or hijack drama?”

Nightsleeper is still available on iPlayer