Some train describers send route setting data as part of there S-Class messages.
- So what is a route?
A route is a physical path between two points over which a train may travel.
- How is a route set?
It depends on the controlling signal box; In an IECC you click the signal you want to set the route 'from' and then click the next signal you want to set the route 'to'.
In a PSB its done much the same but with buttons and switches on a physical panel.
Signalling systems that are fitted with Automatic Routing Setting (ARS) such as London Liverpool Street, the software mimics the actions described above but does not usually require the signaller to physically intervene. There are exceptions to this mostly around permissive working into platforms, and shunt moves where the signaller must always select the route.
Route setting is strictly logical. You can only ever set a route from A to B to C etc. never from A direct to C, nor can you select routes in the 'wrong direction'.
When a route is called it sets off a chain of events in the signalling system. Track sections for the route in question are check to see if they are clear including any overlaps, points are swung and locked into the correct position for the route being selected and once these (and other) conditions are met within the interlocking the route is said to have proved. This does not automatically mean the signal associated with the route will clear (or come off) as there may be further conditions to be met. (Approach, or Lime Street control for example)
A route will normally clear with the passage of a train over that route, or if the signaller manually 'pulls' it. If a route is cancelled in front of an approaching train then the controlling signal will revert to its most restrictive aspect (which will also in turn affect previous signals) but the route will remain locked for 240 seconds to ensure that no points (for example) can be swung in the path of an oncoming train which may not be able to stop at the signal in time. If there is no approaching train then the route is released. Points may or may not auto normalise but this is area specific.
- How many routes can a signal have?
That depends on where you can normally go from that signal. On plain track there may be only one or two routes from one signal to the next, over junctions however and into termini stations there could be two, three or many more.
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