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Inspired by a true story, a runaway train carrying a cargo of toxic chemicals pits an engineer (Denzel Washington) and a conductor (Chris Pine) in a race against time before it derails and destroys a Pennsylvania town.

"This is not a visual effects movie," emphasizes Nathan McGuinness of Asylum VFX, a long-time collaborator with director Tony Scott, who nonetheless supervised the vfx with Asylum colleague Paul O'Shea.

"Tony definitely tries to do everything he can in camera and he will take that to the extreme, which poses challenges for us," McGuinness suggests. "I'm always trying to figure out solutions to not doing visual effects, which is bizarre. So what I have to do is be a supporter of Tony's rather than a problem solver trying to fix it in post using visual effects. And then when I have to I'm under immense pressure to make sure that it matches the photography so it doesn't look like visual effects.

"The challenge was dealing with these trains, which were the third character in this film, and give the impression that this was a non-stop thrill ride from start to finish. From the moment the 777 becomes an un-manned vehicle, it becomes its own creature. With photography and Tony's great visual skill, we were able to make the trains run at a much faster rate than what we were capturing in real life. It makes the audience feel that they are really watching a runaway train."

Indeed, aside from the actors, the main character of the film is a train: two large engines and 25 train cars, out-of-control and speeding toward an inevitable disaster. It is a simple premise, but as a film it begs to be a gritty and unquestionably real experience.

"The long and short of it is that we knew we were going to have to build a CG train," McGuinness admits. "We modeled, textured, lit and animated a photoreal CG 777. Even if Tony wanted a 2D train there were instances when we couldn't manipulate the photography as such; in some cases the train had to go off the rails and we couldn't always do that in 2D so we had to be prepared with a 3D back-up plan."

There were a wide variety of 2D and 3D shots: Replacing the undercarriages of flipping cars, removing camera and crew reflections, adding flying debris, enhancing huge explosions, flatbed removal and often replacement of entire portions of the train as it sped along (behind a blur of organic and man-made structures). But the most crucial and most prominently featured challenges had to do with the faithful recreation of the speeding train and the danger that lay ahead.

"Because the 777 plows through several objects throughout the film, the front end of the engine gets progressively damaged throughout." McGuinness continues. "While on set we photographed and surveyed the four phases of the damaged engine so we could model and texture each phase of the damage. Once the cut was done, we had to go through quite a few shots and replace the front end of the train with the right stages of damage."