A fare to remember: David Nicholls reveals the inspiration for his new BBC drama The 7.39

The script owes more to a Kinks classic than Brief Encounter, the One Day author tells Gerard Gilbert

Wrong signals: David Morrissey and Sheridan Smith in BBC1's 'The 7.39' (BBC)

A man and a woman, quietly dissatisfied with their home lives, are thrown together and begin a relationship while travelling to and from London on a train. Does that sound at all familiar?

However, David Nicholls, author of the best-selling romantic novel One Day and who has now scripted The 7.39 for the BBC, says that his influence was not David Lean's 1945 movie classic Brief Encounter, but the classic Kinks' 1967 ditty "Waterloo Sunset". "The idea came up to do dramas based on songs, and the song we hit on was "Waterloo Sunset" – so that was the working title of it," he says. "I liked this idea of confinement – two people forced to spend time with each other and how over a period of time their relationship might change."

In Nicholls' two-part BBC1 drama The 7.39, David Morrissey plays Carl, a 45-year-old married middle- manager who elbows his way on to the same train each morning – "12 years, no parole… five weeks for good behaviour", as he puts it to fellow commuter Sally (Sheridan Smith), a manager at a health club whose boyfriend wants them to settle down and have babies. Carl and Sally get off to a bad start when they squabble over a seat, but soon they bond over a shared hatred of their daily slog. "The sort of people you see on train stations… the 'hellos' and 'goodbyes'," says Nicholls. "Even though commuting to me seemed quite a gruelling experience it also seemed to have the potential for a bittersweet romantic story.

"Obviously it's impossible to ignore Brief Encounter, but we definitely set out to go in a different direction, to deal as much with the aftermath of the affair. Brief Encounter is a brilliant film but you feel rather as if the cards are stacked against their home life where the kids are always screaming and the husband is always behind a newspaper. I mean I wonder how Brief Encounter would be if you saw Trevor Howard's wife and she was terrific".

Carl's wife in 7.39, Maggie, is played by Olivia Colman, and his home life is depicted as warm, outwardly happy – and predictable. "I can see Carl's view," says Colman. "He's unappreciated. You can why it happens – although he's just forgotten that the life he has is actually very nice."

It's in the second episode that the 7.39 really branches away from Brief Encounter, as the fallout from Carl and Sally's affair is explored in heartbreaking scenes between Morrissey and Colman. "What's beautiful about it is that you get to see the repercussions, unlike Brief Encounter where it's sort of brushed over at the end," says Colman. "It's kind of important also to realise that after this fun romance – these little frissons on a train – there are going to be repercussions."

"It's about wanting what you haven't got," reckons Sheridan Smith, who plays Sally. "She's got this young, puppy-like man at home who wants to get her pregnant, but she wants some wildness in her life. You shouldn't really feel for her but you do… you can't really help who you fall in love with."

While Nicholls says that, as a cyclist and home-working author, he has never commuted in his life, Colman recalls the grimly repetitive journeying of her youth. "I spent years working as a temp and I hated the commute so much," she says. "That's beautifully done where the doors open and they've got fists in each other's backs, and not getting a seat. It used to make me feel angry, so you can really see if there was a little outlet there you might go for it."

"I'm not bright enough to be a commuter," laughs Smith." I worked in a burger bar. And acting is constantly different, but even so, when I was doing Legally Blonde for a year and a half the same thing all the time for 500-odd shows… it's tiring."

Unlike in Smith's Bafta-winning ITV drama Mrs Biggs, in which she played the wife of the recently deceased Great Train Robber Ronnie Biggs and where the train was stationary and rocked to simulate movement, an actual working train was hired in The 7.39. "We had a train at Waterloo Station that the whole crew would get on and it would go wherever… you really see London go by which is really important because it's about that commute," says Smith, although an accident on the line meant that she nearly didn't get to the Bafta ceremony to receive her Leading Actress award for Mrs Biggs – and was the reason she looked so flustered and surprised at winning. "Our producer had to ring ahead to the Baftas, and they said 'don't worry… it's fine'. And because they weren't that bothered whether I was late or not I was convinced I hadn't got it. I made it literally 10 minutes before my award."

One aspect that remains disregarded in the drama is the age-gap between Sally and Carl, for, not to put too fine a point on it, David Morrissey is old enough to be Sheridan Smith's father. "I've always found David handsome," says Smith with a lubricious chuckle. "Anyway, they're kindred spirits and the age-gap doesn't matter." Colman agrees: "It never cropped up in my head because it just seemed they were the characters necessary to help each other out."

"Sheridan is younger than David and I think it is a bit of a cliché," says Nicholls, who started writing for television with ITV's acclaimed relationship drama Cold Feet. "But I think it's more about their situations than their age." Why does he think there are so few love stories on television as compared to, say, thrillers? "Love stories get a bad press really – for a start they're something that men are forbidden to watch and I don't see why that should be the case. I think falling in love is a much more common experience than meeting a serial killer."

'The 7.39' begins on Monday at 9pm on BBC1