What signals are used to indicate 'stop' in railway operations?

In railway operations, red signals are universally recognized as the indication to stop. This is a critical aspect of train control and safety, serving as a clear visual warning to train operators that they must halt their locomotives or trains to prevent accidents or collisions. The standardization of red for 'stop' helps maintain a consistent signaling system across different railway networks, ensuring that engineers can quickly understand and react to the signals they encounter.

In contrast, green signals typically signify 'go' or 'proceed,' blue signals may indicate specific warnings or instructions that do not pertain to a general stop, and yellow signals often serve as cautionary alerts, indicating that the engineer should prepare to stop yet may still be able to proceed with reduced speed. Therefore, among these options, red signals are unequivocally recognized as the sign for stopping, making them the correct response to the question about stop signals in railway operations.