In a fully implemented moveable block signaling system, such as Communications- Based Train Control (CBTC), traditional wayside signals are often greatly reduced or completely eliminated.
This is possible because movement authority, speed commands, and other operating information are transmitted continuously to the train through a digital communication network.
The primary signaling interface becomes the cab display located in the operator's compartment. This display provides all information necessary for safe train operation.
One of the most important items displayed is the movement authority limit. This indicates how far the train is authorized to proceed before it must stop or receive additional authority.
The cab display also shows target speed information. This speed may change continuously based on train spacing, track conditions, curves, station approaches, or operational requirements.
Actual train speed is displayed so the operator can compare train performance against authorized limits. In many systems, distance to the train ahead and train status information may also be displayed.
These continuously updated movement limits function as virtual signals. Instead of viewing a physical signal ahead, the train operator receives a moving authority limit displayed electronically in the cab.
Some moveable block systems still retain limited wayside signals. These signals may be used at interlockings, control points, grade crossings, or during degraded modes of operation.
Fallback signals may also be provided to allow train operation if a communication failure or CBTC system malfunction occurs.
Marker boards, speed signs, station markers, and other permanent trackside signs continue to be used even when traditional wayside signals are reduced.
In a pure moveable block system, the cab display effectively replaces most traditional signal indications by continuously providing real-time operating information directly to the train operator or automatic train control system.