I have been asked the following relating to opposing route locking; I think it is a typical problem for those not used to the implications of sectional release of locking since they are from an environment which is more based on the mechanical locking principles of an entrance signal permanently locking all the points for the entire length of the route until that route is itself normalised.
I have some problems regarding control table. Actually some times I confused direct and indirect interlocking- a route which according to me should be direct was actually indirect, so please help me & explain it with some examples.
First let's go over some basics for background knowledge before trying to answer the actual question asked.
Firstly you must get the concept (which probably differs from the usual case in India and is certainly different from the traditional London Underground approach) of SECTIONAL route release.
When a route is set from entrance signal to exit signal, then all points anywhere along the route itself, its flank and its overlap are initially locked; HOWEVER once the entrance signal has returned to red after the passage of the train and is free of approach locking, then it is only those points which have not yet been traversed which remain locked- the points are "freed-up" behind the train. Locking is held "in front" of the train; points are free "behind" the train; points which were locked initially no longer are. It is when these points were the only thing preventing the setting of the conflicting route that INDIRECT route locking becomes necessary.
Note therefore when considering the locking between 2 routes, the situation is not always symmetrical.
Hence it is important to think about separately:
1. A train passing through "route a" at the time that "route b" is requested,
2. A train passing through "route b" at the time that "route a" is requested,
When doing a Control Table for "route a" you need to think of the signaller attempting to set that one as a new route in the scenario of the historical situation of a train having previously been signalled via "route b" and ask the 2 separate (but related) questions-
i) Is "route b" still locking points in a lie which is incorrect for "route a"? if so the routes are incompatible but it is the point locking which is preventing. The point availability checks for "route a" will not be satisfied and thus route setting is prevented. Route locking is not shown (even if within the circuits / data it is actually included by default),
ii) Should "route b" prevent the setting of "route a"? If it should (because the train on "route b" is still coming towards the train for which we want to set "route a") then we need to show "opposing route locking" on the Control Table, unless the locking is "via the points" as in i) above.
Now to come to the question asked.
DIRECT locking is the situation when the 2 routes mutually lock each other- the point locking imposed by "route a" and that imposed by "route b" is identical within the length of railway which they share. A train using "route a" opposes "route b" until that train gets behind "signal a"; similarly a train using "route b" opposes "route a" until that train gets behind "signal b". This is the symmetrical situation; it should be obvious that there needs to be locking since the point availability test for one would be satisfied in the situation that the other route IS CURRENTLY set; in the absence of different point locking the Control Tables must show the opposing route locking in order to prevent trains being signaled head-on to each other.
INDIRECT locking is the situation when there is some difference in the point requirements between "route a" and "route b" within the length of railway which they share. Hence it may seem that there is no need for opposing locking; at first sight the locking seems to be being provided by virtue of the different point locking; HOWEVER as the first train progresses through its route, then that incompatibility of point locking will subsequently disappear (since the points which were initially locked in an incompatible lie later become free). Therefore there is a need to provide opposing locking in order to prevent the other route setting when this would be undesirable. Note the lack of symmetry in such a scenario; of "route a" CT requires INDIRECT route locking after "route b" then "route b" CT will not feature any reference to "route a" (because the locking of the points the "wrong way"). It is locking that has to be shown because the route WAS PREVIOUSLY SET but wouldn't be needed WHILST opposing route STILL SET but is only necessary after it has been cancelled. Obviously it has to be shown on the Control Tables just like for direct locking but the rationale is for the scenario once the associated point locking has been released by the opposing train being in "mid-route".
The attachment shows a simple layout and tries to show which routes are shown as apposing routes on the Control Tables for just two of the routes- 3A and 8B.
The directly opposing in 3A's CT is 8B. Conversely the directly opposing in 8B's CT is 3A. Symmetry.
To find the all the indirectly opposing routes, look at all the "parallel" signals of the one listed as being directly opposing- once the routes have converged a train on a track circuit could actually have originally come from any of the parallel signals.
Having read this and when you think you have understood it, then look at other posts in this section that relate to opposing route locking, for example this one
PJW
I have some problems regarding control table. Actually some times I confused direct and indirect interlocking- a route which according to me should be direct was actually indirect, so please help me & explain it with some examples.
First let's go over some basics for background knowledge before trying to answer the actual question asked.
Firstly you must get the concept (which probably differs from the usual case in India and is certainly different from the traditional London Underground approach) of SECTIONAL route release.
When a route is set from entrance signal to exit signal, then all points anywhere along the route itself, its flank and its overlap are initially locked; HOWEVER once the entrance signal has returned to red after the passage of the train and is free of approach locking, then it is only those points which have not yet been traversed which remain locked- the points are "freed-up" behind the train. Locking is held "in front" of the train; points are free "behind" the train; points which were locked initially no longer are. It is when these points were the only thing preventing the setting of the conflicting route that INDIRECT route locking becomes necessary.
Note therefore when considering the locking between 2 routes, the situation is not always symmetrical.
Hence it is important to think about separately:
1. A train passing through "route a" at the time that "route b" is requested,
2. A train passing through "route b" at the time that "route a" is requested,
When doing a Control Table for "route a" you need to think of the signaller attempting to set that one as a new route in the scenario of the historical situation of a train having previously been signalled via "route b" and ask the 2 separate (but related) questions-
i) Is "route b" still locking points in a lie which is incorrect for "route a"? if so the routes are incompatible but it is the point locking which is preventing. The point availability checks for "route a" will not be satisfied and thus route setting is prevented. Route locking is not shown (even if within the circuits / data it is actually included by default),
ii) Should "route b" prevent the setting of "route a"? If it should (because the train on "route b" is still coming towards the train for which we want to set "route a") then we need to show "opposing route locking" on the Control Table, unless the locking is "via the points" as in i) above.
Now to come to the question asked.
DIRECT locking is the situation when the 2 routes mutually lock each other- the point locking imposed by "route a" and that imposed by "route b" is identical within the length of railway which they share. A train using "route a" opposes "route b" until that train gets behind "signal a"; similarly a train using "route b" opposes "route a" until that train gets behind "signal b". This is the symmetrical situation; it should be obvious that there needs to be locking since the point availability test for one would be satisfied in the situation that the other route IS CURRENTLY set; in the absence of different point locking the Control Tables must show the opposing route locking in order to prevent trains being signaled head-on to each other.
INDIRECT locking is the situation when there is some difference in the point requirements between "route a" and "route b" within the length of railway which they share. Hence it may seem that there is no need for opposing locking; at first sight the locking seems to be being provided by virtue of the different point locking; HOWEVER as the first train progresses through its route, then that incompatibility of point locking will subsequently disappear (since the points which were initially locked in an incompatible lie later become free). Therefore there is a need to provide opposing locking in order to prevent the other route setting when this would be undesirable. Note the lack of symmetry in such a scenario; of "route a" CT requires INDIRECT route locking after "route b" then "route b" CT will not feature any reference to "route a" (because the locking of the points the "wrong way"). It is locking that has to be shown because the route WAS PREVIOUSLY SET but wouldn't be needed WHILST opposing route STILL SET but is only necessary after it has been cancelled. Obviously it has to be shown on the Control Tables just like for direct locking but the rationale is for the scenario once the associated point locking has been released by the opposing train being in "mid-route".
The attachment shows a simple layout and tries to show which routes are shown as apposing routes on the Control Tables for just two of the routes- 3A and 8B.
The directly opposing in 3A's CT is 8B. Conversely the directly opposing in 8B's CT is 3A. Symmetry.
To find the all the indirectly opposing routes, look at all the "parallel" signals of the one listed as being directly opposing- once the routes have converged a train on a track circuit could actually have originally come from any of the parallel signals.
Having read this and when you think you have understood it, then look at other posts in this section that relate to opposing route locking, for example this one
PJW
PJW