Wigwag is the nickname given to a type of railroad grade crossing signal once common in North America, named for the pendulum-like motion it used to signal the approach of a train. It is generally credited to Albert Hunt, a mechanical engineer at Southern California's Pacific Electric (PE) interurban streetcar railroad, who invented it in 1909 out of the necessity for a safer railroad grade crossing. The term should not be confused with its usage in Britain, where wigwag is generally used to refer to alternate flashing lights, such as those found at modern level crossings.

Soon after the advent of the automobile, speeds were increasing and the popularity of closed cars made the concept of "stop, look, and listen" at railroad crossings a difficult one.

Fatalities at crossings were increasing. Though the idea of automatic grade crossing protection was not a new one, no one had invented a fail-safe, universally-recognized system. In those days, many crossings were protected by a watchman who warned of an oncoming train by swinging a red lantern in a side-to-side arc, used universally in the United States to signify "stop"^{[citation needed]}. This motion is still used today by railroad workers to indicate stop per the General Code of Operation Rules (GCOR) Rule 5.3.1. It was presumed that a mechanical device that mimicked that movement would catch the eyes of approaching motorists and give an unmistakable warning.

Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks on which they run. Track usually consists of steel rails installed on sleepers/ties and ballast, on which the rolling stock, usually fitted with metal wheels, moves. However, other variations are also possible, such as slab track where the rails are fastened to a concrete foundation resting on a prepared subsurface.

Rolling stock in railway transport systems generally has lower frictional resistance when compared with highway vehicles, and the passenger and freight cars (carriages and wagons) can be coupled into longer trains. The operation is carried out by a railway company, providing transport between train stations or freight customer facilities. Power is provided by locomotives which either draw electrical power from a railway electrification system or produce their own power, usually by diesel engines. Most tracks are accompanied by a signalling system. Railways are a safe land transport system when compared to other forms of transport. Railway transport is capable of high levels of passenger and cargo utilization and energy efficiency, but is often less flexible and more capital-intensive than highway transport is, when lower traffic levels are considered.