The Johnsonville line runs through precipitous country, along a river gorge, where double track is not feasible, with grades as steep as 1 in 36. The modernization of the signalling was carried out under the direction of the late Mr G. W. Wyles, then Signal and Electrical Engineer.
Brought into use in July 1938. Four single line sections each about 1.5 miles long. There are intermediate stations at Ngaio and Khandallah and a crossing loop at Wadestown where there is no station.
There are no intermediate block signals, although the scheme is arranged to allow their introduction if necessary. There is, however, a distant signal in the rear of every Home signal. Departure signals are normally at Stop and home signals in the rear are at Caution. The distant signals normally show Clear but should the single line section ahead be occupied by an approaching train they show Caution. Safety points provide against any over-running of the departure signals.
Wadestown is entirely automatic but at Ngaio and Khandallah the mechanical frames still exist, allowing shunting movements or three-train crossings to be made. It is possible for an up train to divide at either Ngaio or Khandallah, with one half proceeding to the terminus and the other half reurning to Wellington. To facilitate this the departure signal is onthe single line; the up loop outlet signal being classed as a starting signal.
Ordinary working at Johnsonville is automatic, but push-button control can be used with suitable precautions.
Working of Wadestown Loop
Each loop can hold a four-car train; ordinarily only the first of two crossing trains needs to stop; the other can run through. During light traffic one and two car trains can cross without stopping. Train stop imposes a speed restriction of 15mph on down trains. This is a necessary precaution because a sharp curve and a tunnel at the northern end of the loop severely restrict the driver's view.
If an up train passes the home signal at yellow and the section towards Ngaio is unoccupied, the points at the ?? end of the loop will reverse and the departure signal will clear. The working in the opposite direction is similar. If another train is being met the first one is held at the departure signal. When the second train arrives the loop points at each end reverses, allowing both trains to proceed. At each departure signal there are two buttons - Cancelling (red) and Clearing (blue). If the cancelling button is used the departure signals goes to stop and after forty seconds the points return to nomal and the opposing train can claim the block. For either button to be operative the track circuit approaching the departure signal must be occupied.
Working of Ngaio & Khandallah Stations
For ordinary movements working is similar to Wadestown. Points other than loop points are rod-worked from the lever frame.
When an up train divides the first half leaves under clear starting and departure signals. After it has cleared the latter, on the single line, the starting signal goes to yellow, allowing the second half to to move out. After forty seconds the loop points return to normal and the vehicles move onto the down main. When the signal box is used to control a three-train crossing or other movement locally, two normally reversed control levers (up and down) are operated in conjunction with the home signal and other levers. Full control is given to the operator after the points, if reversed, have returned to normal. This occurs forty seconds after all levers are put normal and signals are at danger. To restore automatic working the home and control levers are reversed.
Time Control & Block Relay Action
Points cannot restore to normal until signals have been continuously at stop for forty seconds and track circuits concerned have been unoccupied for forty seconds. These precautions ensure that there is no unintended point movement due to momentary loss of track circuit shunt.
The possibility of two opposing trains reaching the station crossing loop area simultaneously has been met by requiring that the single line block section be clear for an additional five seconds after the up train has left it.
Five seconds must elapse after a block section is cleared before any points will reverse to admit a further movement into the section.
If a train is waiting at each end of a section already occupied by a third train, then the following train first obtains the Proceed signal when the section becomes clear. The cancelling buttons can be used to alter this if required.
The signal boxes at Ngaio and Khandallah have track diagrams, at Wadestown there is no signal box and indications appear above the cancelling buttons.
All crossing places are on the train control telephone circuit and when the train controller calls a loop the "T" light indicator is illuminated.
Working Of Johnsonville Terminus
The mechnical frame has been replaced by a small relay interlocking panel, normally unattended. All signals normally show Stop. When an up train obtains the block from Khandallah a route is set to an unoccupied platform line and the relative home signal clears to yellow. A time switch is used to give No 2 line preference from midnight to noon and the No 1 line during the remaining hours. If the line selected by the time switch is occupied, an approaching train is automatically directed to the other track.
A route setting button is placed next to each signal on the diagram. A cancelling button allows signals to be restored to Stop and, after forty seconds, the setting up of another route.
An arriving goods train must stop at the home signal for the guard to press the cancelling button and then a clearing button for the route into the yard. There is the usual forty seconds delay. The low-speed light then clears.
In the first twenty-one months of working with this equipment 416,195 train miles were run and 3,344,362 passengers carried. In two years every point machine at the three loops had made 76,000 movements, with only thirty signal apparatus failures. Six failures were due to obstructions in the points, one to a broken rail, and five to excessively bad weather. Since these figures were compiled the Johnsonville line has been in operation for twelve years without any serious mishap. About 38 million passengers have been carried and the safety record is without blemish.