I have just, when looking for something else, chanced upon the definition that NR has included in their standard relating to Verification and Validation of interlockings which I quote-
A ‘directly opposing route’ to the route under test is a route which:
• Applies in direct opposition to and along the line of the route under test; OR
• Applies in direct conflict in the same direction along the line of route under test (e.g. a shunt signal in advance of the main signal capable of acting as an Independent or Facing Shunt);
AND
• When set would not require any points to be lying in the position opposite to that required by the route under test.
An ‘indirectly opposing route’ to the route under test is a route which:
• Applies in the opposite direction to the route under test; AND
• When set does not ‘directly oppose’ the route under test because it requires at least one set of points in the opposite position to that required by the route under test; AND
• When normalised, after a movement has entered the route and passed clear of and released from locking all points not set in the position required by the route under test, leaves the remaining route locked portion of the opposing route effectively ‘directly opposing’ the route under test as defined above.
A ‘directly opposing route’ to the route under test is a route which:
• Applies in direct opposition to and along the line of the route under test; OR
• Applies in direct conflict in the same direction along the line of route under test (e.g. a shunt signal in advance of the main signal capable of acting as an Independent or Facing Shunt);
AND
• When set would not require any points to be lying in the position opposite to that required by the route under test.
An ‘indirectly opposing route’ to the route under test is a route which:
• Applies in the opposite direction to the route under test; AND
• When set does not ‘directly oppose’ the route under test because it requires at least one set of points in the opposite position to that required by the route under test; AND
• When normalised, after a movement has entered the route and passed clear of and released from locking all points not set in the position required by the route under test, leaves the remaining route locked portion of the opposing route effectively ‘directly opposing’ the route under test as defined above.
PJW