Bradshaw Disposal Services has a nuclear bomb made in Russia to transport, and an employee (named Henry Bradshaw) decides to save money by concealing it on a freight train. This train is also loaded with hazardous and flammable chemicals, including metallic sodium, which spontaneously ignites on contact with water. The train suffers a brake failure and becomes a runaway heading for Denver. John Seger, a National Transportation Safety Board (NTSB) investigator, boards the train and with the assistance of the railwaymen tries various ways to stop the train. Several ideas are tried, such as coupling a following train to the caboose (the coupling mechanism on the caboose breaks, which also results in the death of one of the train's crew members), a derailing attempt (after it is revealed the catastrophe the chemicals would cause if ignited) in which a helicopter narrowly avoids being hit, and finally, an attempt at manually activating the brakes (via hitting a part of the engine mechanism with a wrench). The final attempt works successfully but is short-lived. The following rescue train, unaware of the freight's slowing, speeds forward and crushes the caboose (killing an injured crew member in the process). The force disengages the brakes, this time for good, causing the train to speed up once again. Meanwhile, Denver residents are struggling to collect their families and then leave town, despite rioters, looters, and gridlock. Realizing that there is no way to stop it, John and the engineer (who was initially reluctant) abandon the train by jumping off before it can speed up too much.