Diverging Routes

Signalling Diverging Routes - A Conundrum

New Zealand railways use speed signalling. A discussion of speed vs route signalling is always interesting, particularly when there are only one or two possible routes. In 1969 NZR signal engineers appeared to change the way they applied speed signalling principles at loop exits. Then in 1998, when CTC was installed on the Johnsonville branch, a non-standard low-speed indication was used at loop entry.

On this page I use examples from Ngaio to demonstrate the conundrum of signalling diverging routes.

Home > Johnsonville > Diverging Routes

The single unit Up Departure From Up Main (left) and Up Departure From Down Main (right) at Ngaio. The first of the refurbished English Electric EMUs, which is operating a down service, approaches the station. Note the grade.

Signalling Diverging Routes

"Loop" is a convenient way to describe the track configuration at Ngaio. Strictly, the "Loop" is designated the Up Main.

Loop Exit

The Up Departure signal at Ngaio displayed Medium Speed until 1969, when the indication was changed to Normal Speed. There is only one possible route beyond the departure signal (ignoring shunt moves) and trains are travelling slowly, probably at about the safe speed for the points. Therefore a Normal Speed indication is probably more appropriate. I can only speculate that Medium Speed was used in the original installation because trains took the diverging route - route signalling.

My Loop Exit page gives more examples of the differing ways loop exits have been signalled.

Note that in 1938 the signal was classified as a Starter. This was part of the facility for dividing trains and is not relevant to this page.

Loop Entry

Until 1998 loop entry at Ngaio and Khandallah was signalled conventionally, although as the station tracks were signalled for one way working there was no Normal Speed indication. In 1998 CTC was installed and the Up and Down Mains were re-signalled for bi-directional working. There were now two possible routes - the diverging one to the Up Main and the straight through to the Down Main. To install full CTC would have required large alterations to the existing interlocking.

The solution? Under normal operation the diverging route is the usual route - the Down Main is rarely used by Up trains. An illuminated low-speed light indicates that the points are correctly set and locked for the Down Main and that the track is unoccupied. The latter is contrary to standard practice. Prior to 1969 the low-speed authorised a move to either the up or down main.

Last Updated: Saturday, December 31, 2005