Diagram A   A down train is at Awarua Street station and is scheduled to cross an up service at Ngaio. The intermediate is displaying green. The up train is running late and has not yet claimed the Wadestown - Ngaio section.

Diagram B    The down train proceeds to the Down Main at Ngaio and claims the Ngaio - Wadestown section. The up train has now arrived at Wadestown but cannot proceed. If the crossing is made at Wadestown the Up train will be further delayed. If, instead, signal 4LA is put to Stop there will be delays while timers run down and the block is cleared for the Up train.

Diagram C    A better solution? The down train waits at green intermediate signal 593 until it goes to yellow. This is the SLA Double Yellow indication, whic has been retained under CTC, and tells the driver that the opposing train has claimed the Wadestown - Ngaio block. The down train can now proceed and berth on the Down Main at Ngaio.

This excerpt from the North American GCOR (General Code Of Operating Rules, Fourth Edition April 2000) is of interest, although of no relevance to New Zealand.