Filmed in December 1971, the first Christmas for Peter Cushing since the February 14 death of his beloved wife Helen. Christopher Lee's family made it as warm an affair as possible for Cushing, who would grieve for his lost wife for the remainder of his life, often playing roles that mirrored his own sadness. Despite the freezing working conditions and "abominable" food, this film provided one of the few co-starring roles where the two actors get to work in unison, rather than opposing one another, with Cushing getting the most amusing lines.
Peter Cushing arrived in Spain for filming and immediately told producer Bernard Gordon that he could not do the picture, as he felt it was too soon after his wife's death. Christopher Lee convinced Cushing to stay on by reminiscing with him about the previous films they'd worked on together, much to the relief of Gordon.
Most of the film was shot without audio recording; the soundtracks and dialog were all added in post-production. Christopher Lee, Peter Cushing and Telly Savalas provided their own voices for the film's English-speaking version.
During the production there was only one set available for the interior of the train cars. All of the scenes for each train car had to be shot at once and then the set would have to be reconstructed for the next train car.
Telly Savalas was paid $ 22,000 for his short stint on this movie in Madrid. Producer Bernard Gordon was delighted to get him for such a low price; Savalas was equally delighted, as he used the cash to get a seven-year lease on a West End apartment for his then girlfriend.
The train interior sets and the train model used for the exterior shots were the same sets used for Pancho Villa (1972), made by the same producer and director, which had just finished production and which also featured Telly Savalas.
It has been told that Peter Cushing was so devastated because of his wife's recent death by the time the film was shot that he would suffer from night panic. In order to make his nights easier, so he would not be alone when the attacks happen, Christopher Lee used to sleep in his same bed.