North American railroad signals

North American railroad signals generally fall into the category of multi-headed electrically lit units displaying speed-based or weak route signaling. Signals may be of the searchlight, color light, position light, or color position light types, each displaying a variety of aspects which inform the locomotive operator of track conditions so that they may keep their train under control and able to stop short of any obstruction or dangerous condition.

Source: Wikipedia — North American railroad signals (CC BY-SA 4.0)