London office worker Benjamin Fowler is teased by his workmates Smith and Farquhar because he plans to spend the bank holiday weekend train spotting in Staffordshire. He tells them that the highlight of his weekend will be a journey on the last train through the soon to be condemned Harecastle Tunnel.

On the train that evening, Benjamin shares a compartment with four soldiers who joke about train spotters. Later in the buffet car, Benjamin shares a table with Mr and Mrs Dyson and Megs, Mr Dyson's sister. Mr and Mrs Dyson are fascinated by Benjamin's enthusiasm for the railways, but Megs remains uninterested and asks to be taken back to her carriage. After Mr Dyson and Megs leave, Mrs Dyson tells Benjamin she never really loved her husband and he should make the most of his life while he is young. A drunken army officer joins Benjamin. He tells Benjamin that he dislikes the army - especially his own men.

Arriving in Staffordshire, Benjamin checks into a bed and breakfast owned by Adam Coulson, a former railway worker. Coulson has a large collection of railway paraphernalia, which Benjamin is impressed by. Benjamin tells Coulson that he would love to work on the railways, but his father insisted that he got an office job. Coulson tells Benjamin that he believes the railway is in decline because it employs too many black workers. Coulson asks Benjamin if he would take his grown-up son Jackie with him on the journey through the tunnel in a bid to encourage him to take an interest in the railways, Benjamin agrees.

Arriving at the station, Benjamin buys his ticket and Jackie makes suggestive comments to the man in the ticket office. Benjamin notices the man behind him in the queue buys two tickets although he is alone. While waiting for the train, Jackie goes into the toilet where he meets another man. He tells Benjamin that he will not be going on the train after all and leaves with the other man. On the train Benjamin strikes up a conversation with the man from the queue, a former railway engineer called Judge Grayson who explains he bought a spare ticket for a dear friend. As the train journeys through the tunnel, the passengers sing 'for she's a jolly good tunnel'. Grayson invites Benjamin to his house the next day.

Arriving at Grayson's house, Benjamin meets Beatrice, Grayson's teenage daughter who shows him into Grayson's room. The room is a mock-up of the interior of a signal box. Grayson teaches Benjamin how the controls work and the two men pretend they are manning a busy crossing. Out of the window, Benjamin sees Beatrice in a clinch with Grayson's gardener. Grayson tells Benjamin to ignore them and keep his eyes on the signals. Grayson asks Benjamin if he would deliver the spare ticket to Bob Phillips upon his return to London. He explains that he and Phillips were good friends who fell out over a personal matter. When Beatrice hears that the spare ticket is for Phillips, she becomes angry.

Returning to London, Benjamin heads for Phillips' house. Phillips, a concert violinist, lives with his disabled wife, who tells Benjamin that the railways are Phillips' passion. Upon receiving the ticket, Phillips tells Benjamin that he would like to make up with Grayson and maybe visit him. Phillips picks up a pile of papers and out of them falls some photographs of Beatrice. Mrs Phillips enters the room and stares in silence at the photographs.

At work the following day Smith and Farquhar discuss their disappointing weekends. Smith had a disastrous date with a woman and Farquhar failed to have a round of golf with their boss. Farquhar says how much simpler life would be if they had mundane lives like Benjamin's. Benjamin sits in silence.