05-09-2014, 11:25 AM
Update
For schemes using Axle Counters and SSI/SSI-Type interlockings Only:
Network Rail HQ is encouraging projects to apply for a TNC/derogation to allow unconditional TORR owing to the saving in design and testing time it should provide.
The underlying reason is historical: When TORR was first provided in Relay interlocking there was a concern that failure situations might put a signal to red in front of a train. As Comprehensive Approach Locking (CAR) was provided anyway, they conditioned TORR on the CAR to avert that risk.
When TORR started being applied to SSI, and CAR no longer provided universally, someone felt it necessary to still have an additional condition on TORR. As the SSI program inherently conditions TORR with approach locking (including CAR if provided) the track sequence condition seems to stem from a lack of understanding of the SSI program.
Implemention of unconditional TORR, on some recent projects I'm aware of, has been limited to specific routes where the track sequence is not available (usually routes with few tracks involving permissive working). I gather that NR's standards freeze may be a factor in delaying wider use of unconditional TORR.
For schemes using Axle Counters and SSI/SSI-Type interlockings Only:
Network Rail HQ is encouraging projects to apply for a TNC/derogation to allow unconditional TORR owing to the saving in design and testing time it should provide.
The underlying reason is historical: When TORR was first provided in Relay interlocking there was a concern that failure situations might put a signal to red in front of a train. As Comprehensive Approach Locking (CAR) was provided anyway, they conditioned TORR on the CAR to avert that risk.
When TORR started being applied to SSI, and CAR no longer provided universally, someone felt it necessary to still have an additional condition on TORR. As the SSI program inherently conditions TORR with approach locking (including CAR if provided) the track sequence condition seems to stem from a lack of understanding of the SSI program.
Implemention of unconditional TORR, on some recent projects I'm aware of, has been limited to specific routes where the track sequence is not available (usually routes with few tracks involving permissive working). I gather that NR's standards freeze may be a factor in delaying wider use of unconditional TORR.