Some are common, others are unique to a specific railroad. The most constant signals and the easiest to understand are single-head block signals. Red means stop; green means proceed, and yellow means caution or approach, usually indicating that the next signal is red.

In the world of railroad signaling and operations, a yellow flag (or yellow light/signal) is a "Caution" indicator that typically means "Proceed, but be prepared to stop." When an engineer sees a yellow flag displayed by the side of the track or held by a worker, it serves as a warning that the next signal is likely "Red" (Stop) or that there is a "Slow Zone" ahead due to track maintenance or an obstruction. In 2026, modern Positive Train Control (PTC) systems display these "yellow" warnings directly on the engineer's computer screen, but physical flags remain a critical backup. A yellow flag tells the engineer to immediately begin reducing the train's speed so they can bring the massive weight of the train to a complete and safe halt before the next signal. In some specific rail systems, a yellow flag might also indicate that the train is being diverted onto a secondary track or siding. Essentially, yellow is the "heads-up" signal that allows for the long braking distances required by heavy freight and high-speed passenger trains, ensuring the transition from full speed to a stop is controlled and safe.