YLP Wrote:[I have several queries on the Notes:
1. Bite 6 Opposing Route locking (Point 8 & 9)
- I am not too used to using the bracket for the time released statement, can you explain more about the meaning of the bracket?
- For Point 9, why the T.C. to be enclosed withing a bracket is different for non-permissive opp. route & permissive opp. route?
- Can I omit the use of bracket like the old style C.T. samples?
The opening position of the bracket is significant in determining the extent of the opposing route locking that is bypassed when the train has come to a stand. Generally the train needs to be timed on the last track, but if there is a rational reason- such as a platform- for a train to come to a stand prior to the last track then the timing may be on two or more combined. For obvious reaons any of the tracks on which the timing is performed must be "within the brackets" in the route locking column, but potentially there may be more than that. SSI generally only bypasses the bare minimum whereas RRI generally bypasses ALL the tracks back to the opposing signal; i.e. a track occupied for time overrides all the route locking (but NOT the actual Route Normal).
One can argue that SSI is therefore slightly "safer" in this regard; even if there is a rightside track failure on the timimng track, the route locking prior to this is still tested to be free, whereas in RRI it is not- however I think the real reason for the difference is that SSI relies upon the sub-routes to carry the approach locking of the opposing entrance signal whereas RRI does include this explicitly (to cover timing issues) despite it being in the route locking cascade relays as well. Hence actually what a Control Table shows is dependent on technology; I generally default to showing SSI as it is the default technology on NR BUT EACH CANDIDATE CAN FOLLOW THEIR KNOWN PRACTICES.
In SSI therefore the brackets are 100% essential. Less so on RRI since the unwritten convention is that the timing tracks always bypass all the route locking in the preceding column; however I'd still show as it is more explicit and can certainly give clarity where entry spans a couple of lines of text etc.
Re your second query.
If a train has come to a stand it will have timed out its overlap but the route locking under the train will still be locked. This should prevent an opposing Main or Warning route being set (being non-permissive the aspect won't be able to clear hence to allow the route to set would be a waste and also mislead the signaller). However a Call-on or Shunt (assuming permissive) should be allowed to set despite the locked sub-route (i.e. bypasssed by the track timer) since the aspect will be able to clear to allow a second train / loco to attach to the previous train.
However in most cases we wouldn't want to
simultaneously allow another train to be routed on to the first train from the opposite direction. Sometimes historically we did allow this (era of parcels traffic, loco changes, adding and taking off vehicles at main stations when an express stopped)- referred to as
"topping and tailing". Only provided where necessary for traffic; there is always the risk that the track occupancy may not be a train at all but a track failure and having trains routed head-on albeit only on PL aspects is still not good!
Therefore the significance of the bracket position- does the track occupancy bypass just the locking for that track, or for all the tracks prior to it as well; this determines whether can "top'n'tail".
Also remember the mere fact that a Call-on has been able to be set suggests that the track timer on the berth exit track will already be occupied before the second train even enters the route; indeed it is very likely to have reached the time out value well before the second train gets anywhere near it (because the first one has been sitting there!). Hence if you bridge out all the route locking sections due to the timer having completed, nothing would stop the setting of a permissive route from the other direction; there is no risk of that happening with a non-permissive route as the exit track route locking is not itself bypassed by the track timer, so the simpler logic suffices.
Do note the use of passenger permissive platforms, especially for platform sharing by different trains, is looked upon less favourably nowadays; see GK/RT0044 for the relevant additional locking.