Director Jonathan Gershfield’s has annoyed the country’s train drivers with his new film comedy.

People diving under trains is not exactly a likely comedic theme, but this grim subject forms the plot of new British movie Three and Out.

In it, Mackenzie Crook is Tube driver and aspiring author Paul Callow, who accidentally runs someone down with his train. Unluckily, a few days later he has another “one under”. After finding out that on the London Underground, if you have “three under” in a month you can leave with 10 years’ salary, he sets about trying to find his third casualty.

Unsurprisingly, the subject of the film was a cause for concern, particularly for the train driver’s union Aslef. Writing on its website, general secretary Keith Norman said the plot was “insulting and foolish”.

But director Jonathan Gershfield argues that the best comedy comes out of tragedy.

“Comedy should have some depth to it,” says the 50-something director, originally from Southgate, North London. “Most comedies are a bit trivial or frivolous. I knew I wanted do something that would make people think. This film definitely starts off as a broad comedy and it changes gradually and it becomes a poignant drama. It deals with some serious issues like assisted suicide.

“The complaints came from people who haven’t seen the film. If Norman had seen the film, he wouldn’t have the concerns that he had. It’s actually a very sensitive film about friendship and all sorts of big human issues.”

Three and Out is Gershfield’s debut movie, having worked in advertising and TV comedies like Big Train, Dead Ringers and Twisted Tales.

Gershfield, who grew up going to Southgate and District Reform Synagogue, says he feels privileged to have enlisted a stellar cast including Crook; Star Trek’s Colm Meany; Gemma Arterton, who has just been announced as the next Bond Girl; and Imelda Staunton, who insisted on rewriting some of her lines to make her character more believable.

There are also cameos from Sir Antony Sher and Kerry Katona (who Crook describes as the next Barbara Windsor). “Mackenzie was an absolute joy to work with — one of the most easy-going actors I’ve had the pleasure to work with,” says Gershfield, who recounts the scene where Katona dangles him off a bridge and Crook only admitted to breaking a rib after shooting had finished.

“We were looking for someone who was going to be convincing as a train driver and looked as if they were a thinking person — someone with literary ambitions. As soon as his name was suggested, I thought he was ideal for it and as soon as we sent him the script, he loved it.”

Three and Out is released on Friday April 25