In a Cab Signal System (CSS) territory, what governs movement between fixed signals?

In a Cab Signal System (CSS) territory, movement between fixed signals is governed by cab signals. This system utilizes in-cab displays that relay signal aspects directly to the train crew, providing real-time information about the status of signals ahead. The cab signals are designed to enhance safety by ensuring that the engineer is aware of upcoming signals as they approach them, thus allowing for prompt and appropriate responses to any restrictive aspects.

Cab signals are integral to the safe operation of trains in CSS territory, offering a continuous flow of information rather than relying solely on external fixed signals, which could be obstructed or missed. By using this system, movements can be controlled more efficiently, as the cab signals can provide indications based on changes in the track ahead, including sudden stops or speed restrictions.

The other choices do not accurately represent the governing mechanism within CSS territory. Crew discretion and verbal permissions may be applicable in other contexts but are not the primary means of authority in a cab signal system. Distance markers do not provide the necessary real-time information required for safe train operation between signals. Thus, cab signals are essential for ensuring that train crews have the most current and accurate data when making operational decisions.