The Railway Family

From Wiki and Wiki:

During World War II, Eric Lomax (Colin Firth) is a British officer who is captured by the Japanese in Singapore.  The story follows the adventures of his children, who are forced to move with their mother (Dinah Sheridan) from a luxurious Edwardian villa in the London suburbs to "Three Chimneys", a house near the fictional 'Great Northern and Southern Railway' in Yorkshire.

Lomax is sent to a POW camp, where he is forced to work on the Thai-Burma Railway.  The three children, Roberta (Bobbie) (Jenny Agutter), Phyllis (Sally Thomsett) and Peter (Gary Warren), find amusement in watching the trains on the nearby railway line and waving to the passengers.

During his time in the camp, Lomax is tortured by the Kempetai primarily for building a radio. The children become friendly with Albert Perks (Bernard Cribbins), the station porter, and with the Old Gentleman who regularly takes the 9:15 down train.

Meanwhile, to earn money to survive during her husband's absence and still suffering the psychological trauma of his wartime experiences, Mother writes and sells stories to magazines about her husband  and best friend Finlay (Skarsgård), finding and confronting one of his captors.

After many adventures, including saving the lives of dozens of passengers by alerting a train to a landslide and rescuing a Russian dissident, Mr Szczepansky, the family returns to the scene of Lomax’ torture and manages to track down his captor, Japanese officer Takashi Nagase (Sanada), who appeals to the Old Gentleman for his help, "in an attempt let go of a lifetime of bitterness and hate".