"Unstoppable" was in development for an extended period, starting in 2004, with actual filming not beginning until August 31, 2009. At various points, Robert Schwentke and Martin Campbell were attached to direct.
Ironically, a train used in filming accidentally derailed in Bridgeport, Ohio on November 21, 2009 while being shot for the production. No one was injured in the incident, but production was halted for the remainder of the day.
Twentieth Century Fox executives asked Denzel Washington to shave $4 million from his standard fee of $20 million. (They also asked director Tony Scott to cut $3 million from his usual $9 million fee.) Citing frustration with the lack of a start date, Washington withdrew from the film. Fox then came up with an as yet undisclosed enticement package, purportedly including a revised script, to bring Washington back on board two weeks later.
Although it was sometimes hard to tell during the sugar puffed cereal/potato flake storm scene, Chris Pine performed all of his own stunts. Denzel Washington had seven stuntmen, one for each day of live shots on running trains. In addition to insurance concerns, according to Tony Scott, "D's got a fear of heights, and I had him up at 25 feet on a 50 mph train, which was no easy task." When you do see Washington up on top of a tanker car, that's really him, though, not CGI.
Real life train engineer Jess Knowlton served as a technical advisor to Denzel Washington. Knowlton's daughters actually work at Hooters, which is how Washington's Frank Barnes character's daughters wound up being similarly employed.
The locomotives used in the movie were leased General Electric AC4400CWs from Canadian Pacific (4 units) and EMD SD40-2s from the Wheeling and Lake Erie dressed up as the fictional Allegheny West Virginia Railroad.
The "Stanton Curve" featured in the film is an actual rail line in Bellaire, Ohio. The line runs on a historic stone viaduct after crossing the Ohio River from West Virginia. However, the extremely dangerously-placed oil/chemical storage tanks beside the curved track do not exist and have been added in by CGI to increase the sense of danger.
A self-confessed acrophobic, Denzel Washington reluctantly performed the stunts where actually he runs along the top of the speeding train. What made the stunt even more hazardous was the fact that the train was empty, thus causing the individual cars to rock more violently. Even though Washington was "wired" as a safety precaution, the leaps from car to car presented a potentially hazardous sequence (interview: Breakfast: Episode dated 26 November 2010.
The two maps of the rail line shown on the news are of real Pennsylvania counties: The First map has Union and Snyder on the left, and Northumberland, Montour and Columbia in the center and Lycoming on top as well as Schuylkill on the bottom. The Second map has Cameron on the top left, Clinton in the center, Centre on the bottom and Lycoming on the right.
The film is inspired by the "Crazy Eights" unmanned train incident in 2001. The train, led by CSX Transportation SD40-2 #8888, left its Walbridge, Ohio, rail yard and began a 66-mile (106 km) journey through northwest Ohio with no one at the controls, after the engineer got out of the originally slow-moving train to correctly line a switch, mistakenly believing he had properly set the train's dynamic braking system, just as his counterpart in the movie did. Two of the real train's tank cars also contained thousands of gallons of molten phenol, similar to the fictional train in the film.
The only siding long enough to allow 1206 to dodge the oncoming runaway is a "rip track". A rip track is a siding on which equipment can be parked for maintenance/repairs that don't require it to be taken to the shops. "Rip" is an acronym for "Repair In Place".
There are three occasions where it seems a line has been either cut or overdubbed. Both Gavin and Will both say "Goddamn", but the word is either cut totally, like in Will's case, or the word God has been cut, like in Gavin's case. However, there is the case of Ned. When Ned says "You guys are always screwing up.", The words "Screwing up.." seem to replace Ned saying "You guys are always 'fucking up'. These lines may have been cut/re-edited to help the movie have a PG-13 rating, instead of an R rating.
The trivia item below may give away important plot points.
Dynamic brakes work by switching the traction motors that drive the axles to act as generators. The current produced is dissipated as heat in a resistor grid located atop the locomotive. When Will attempts to brake the train coming into Stanton, the traction motors overheat and burn out - this is the flashes of fire under the train. The "independent" brake is the locomotive's own air brake, which still functions (by clamping against the wheel treads) after the dynamic brake is burnt out.