Which type of stop arm can never be lowered?

A fixed stop is designed to remain in a constant position and cannot be adjusted or moved. Unlike adjustable and movable stops, which can be configured for various conditions or operational needs, the fixed stop is permanently affixed to a specific location to ensure that it consistently provides a designated stopping point. This characteristic is crucial for specific scenarios, such as ensuring that trains halt at a precise location for safety and operational efficiency.

In contrast, adjustable and movable stops can change location or position based on the requirements of the rail operation or safety protocols. Retractable stops, while capable of being lowered and raised, also serve different purposes and flexibility that a fixed stop does not possess. Therefore, the essential nature of a fixed stop, remaining static and not being able to lower, distinguishes it clearly as the correct answer.