Adam Lucke's Highway Engineering Blog: Rail Crossings

Rail crossings, formally referred to as "level crossings," are a common type of traffic control feature. This is where two modes of transportation intersect with one of them being a train rail line and, most typically, a street.

Typical arrangement of modern level crossing safety system

Level crossings were originally manned by a man in a booth who, upon the approach of a train, would either wave a flag to warn of the approach or clear livestock from the tracks. The first mechanical warning gate systems were more concerned with clearing and securing the track from livestock and so would incorporate an actual gate to physically prevent animals from occupying the tracks.

Example of older-style level crossing

As the automobile came to dominate the roadways, the mechanical warning system became focused on drawing attention to the motorist of approaching train traffic and relying on the discretion of the driver to not occupy the track area.

Signal styles vary from county to country as to levels of traffic impediment, from a simple manually operated gate to much more complex systems.