20-04-2014, 02:23 PM
You will probably find better explanations than I can give but this is how I think of it:
You want to set R371D(S), and need to ensure there are no other routes set which would cause a collision.
So we look at the other routes which might already be set -
* Directly opposing is where the other route is the reverse of 371D, thus no points calls already prevent the opposing route from being set. We use tracks clear conditions to ensure the opposing train is beyond signal 371.
*Indirectly opposing is where the other route starts from somewhere different; as this train moves through its route points become free to move because route locking only holds the route in front of the train.
Think about a train moving through R378B(S), once it clears track DR, P235 become free to move as the route locking clears behind the train. At this point the opposing move has not passed Signal 371 so we need to add opposing locking controls.
Now think about a train moving through R382A, it needs to travel until it clears track CF, before P234 are free to move and it's possible to set R371D, however the train has already passed signal 371 so it is no longer opposing R371D(S). Therefore it does not need to be included in opposing locking in this case.
Does that help? Perhaps PJW can provide a better explanation?
You want to set R371D(S), and need to ensure there are no other routes set which would cause a collision.
So we look at the other routes which might already be set -
* Directly opposing is where the other route is the reverse of 371D, thus no points calls already prevent the opposing route from being set. We use tracks clear conditions to ensure the opposing train is beyond signal 371.
*Indirectly opposing is where the other route starts from somewhere different; as this train moves through its route points become free to move because route locking only holds the route in front of the train.
Think about a train moving through R378B(S), once it clears track DR, P235 become free to move as the route locking clears behind the train. At this point the opposing move has not passed Signal 371 so we need to add opposing locking controls.
Now think about a train moving through R382A, it needs to travel until it clears track CF, before P234 are free to move and it's possible to set R371D, however the train has already passed signal 371 so it is no longer opposing R371D(S). Therefore it does not need to be included in opposing locking in this case.
Does that help? Perhaps PJW can provide a better explanation?