scope:

Introduction

A common requirement in signalling a station area is for different classes of routes from a signal over the same line to a common destination.

Main route: Full overlap is provided, main aspect clears with normal approach release conditions if required.

Warning route: Restricted overlap or no overlap is provided, main aspect clears with restricted approach release conditions.

Call-on route: No overlap is provided, the associated position light aspect clears with restricted approach release conditions.

On the Signaller's Panel, the main and warning routes share a common red entrance button and the main and call-on routes share a common red exit button. To allow the Signaller to select the required class of route, an additional exit button, coloured yellow, is provided for the warning route, and an additional entrance button, coloured yellow, is provided for the call-on route. (Previous practice was for the main and callon routes to both use the red exit button with the class of route selected depending on the state of the exit signal's berth track section - see Section 3.6 and SSI8003-83 for more details.)

Alternative methods of selecting warning routes by overlap availability, rather than by separate button, are covered by SSI8003-79 Section 6.

After a warning route has been set, there are certain circumstances in which it may be allowed to "step-up" to a main route. This occurs if a forward route over the overlap for the main route has been set and the overlap track sections are clear and points detected, and the signal is still being held at red by the approach release conditions for the lower class of route.