How does a penny left on the track derail a train?

Category: Physics
Published: December 8, 2012

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

A penny left on a track does not typically derail a train. A train speeding along its track is a very heavy object with an immense amount of momentum. The penny is simply too light to do much of anything. It is flattened or knocked out of the way by the train. The basic principle at work is the conservation of momentum. Objects with low momentum (light objects going slow) cannot do much to heavier objects because they have little momentum to give to the other object. This is why specks of dust don't knock over motorcycles, ducks don't capsize cruise liners, and mosquitoes don't detour trucks. Flattening pennies using trains is still dangerous though; to the people placing the pennies. A search of newspapers reveals several cases of pennies failing to derail trains but succeeding in getting the penny placers killed in the process. Trains aren't invincible though. A car, truck, or even a brick left on the track can lead to derailment. According to the Federal Railroad Administration, 1.4% of train derailments from 2009-2012 were caused by objects on the track. This group of causes includes snow, ice, and coal. The vast majority of derailments are caused by railroad equipment failure. For your own safety and the safety of the train, never place anything on the tracks.

A penny left on the tracks is too small to derail a train. Don't try this out, though, as many people have been killed in the attempt. Public domain image, source: U.S. Farm Security Administration.

Topics: derailing a train, momentum, penny, penny on track, train