Automatic Signalling on the Midland Line

 

DXC 5039 passes the up arrival and down departure automatic signals at Moana.The locomotive engineer of the coal train can be seen in the background preparing to set the points to the loop so they can depart. 11/09/2026

I've recently taken up a short stint working on the West Coast of the South Island. On my travels I've visited lines and locations I haven't seen before. Like the Main Trunk through the central North Island, the electrified suburban network in Wellington or the Central Otago Branch, the West Coast holds a special place for railfans. This was because the West Coast was one of the last places in New Zealand to regularly run steam locomotives on goods trains, had branch lines like the famous Rewanui and the Conns Creek Branches serving remote coal mines, ran through the stunningly beautiful Buller Gorge and across the road rail bridges on the Hokitika Branch. The West Coast is deeply embedded into New Zealand railfan folklore. 

The railway that crosses the Southern Alps to link the east and west coasts of the South Island is called the Midland Line. This line has always been an important part of the New Zealand rail network, and today it is no different. New Zealand’s premier passenger train, the Tranz Alpine shares the rails with heavy coal trains from Mai Mai, Reefton and Ngakawau, milk wagons from the Hokitika line and a steady stream of logs from Greymouth. This traffic has resulted in the upgrading of track and replacement of numerous bridges, however for all the traffic and upgrades, it continues to operate using a signalling system that is a throwback to a past era.

First introduced to the Midland Line in 1923, Single Line Automatic Signalling remains in use between Rolleston and Arthurs Pass and Otira and Stillwater on the Midland Line. The section between Arthurs Pass and Otira including the Otira Tunnel is controlled by Centralised Traffic Control. 

Single Line Automatic Signalling is a railway signalling system that uses track circuits and axle counters to automatically control trains on a single track.

To simplify it to my level, the single track is broken down into smaller sections called blocks. The track circuits and axle counters detect when a train enters or leaves a block and the automatic signals change colour to green - proceed, yellow - caution prepare to stop or red - stop, depending on the occupancy of the block ahead. This system stops trains from entering a block that is already occupied.

Kokiri - Moana signalling and interlocking diagram.

Between each station their intermediate signals. These signals act as dividers and break up the sections of track between stations into smaller blocks and indicate the occupancy of the blocks ahead.

Intermediate signals between Kokiri and Moana.

Intermediate signals between Jackson and Rotomanu. Axle counters are installed in this location.

Intermediate signals also act as advanced warnings for trains as they approach stations. An intermediate signal showing a yellow caution aspect before a station can indicate that the arrival signal at the station ahead is red because there is a train on the main at the station or the points are aligned for the loop.

The vast majority of crossing loops on the automatic signalling sections of the Midland Line have manually operated points and the pair of arrival and departure signals at each end. I don't envy the locomotive engineers that need to get out of their warm dry cabs in the winter at Cass or the rain at Moana to change the points to enter the loop or depart on the main.

Rotomanu Signalling and Interlocking Diagram.

The crossing stations are interlocked meaning the points are protected by the arrival signals that have 'L' lights - L standing for loop.

The Down Arrival signal at Rotomanu with short range 'L' signal.

The points are electrically connected to the signals so that when the points are set for the loop at either end, both arrival signals go to stop and the short range 'L' is illuminated at the end that the points are set for the loop. Using the 'L' light, trains can enter the loop.

Down Departure Signal, loop points switch lever and releasing switch box at the down end of Kokiri Station.

On the Midland Line, when a freight train is crossing another freight train, the down train (west bound) always takes the loop. Passenger trains including the Tranz Alpine almost always remain on the mainline. 

The westbound Tranz Alpine passes a work train in the loop at Moana. 13/09/2026.

A releasing switch is provided near the points lever. If the block ahead of the train is clear, activating the releasing switch will set the departure signal to proceed. Once the proceed signal comes up, the locomotive engineer can set the points allowing the train to depart the loop. 

Releasing Switch Box at the down end of Kokiri station.

If a train arrives at a crossing loop and no crossing will take place, the approach and departure signals will be at proceed unless the blocks ahead are occupied.

When trains arrive at stations where crossings have previously occurred, the Locomotive Engineer may get a stop indication at the arrival signal and will need to manually set the points back for the main line. The arrival signal should then return to proceed. If it remains at stop the locomotive engineer can pass the red signal at low speed ensuring that all points are set for the main and there are no obstructions on the line.

Because theres traffic going in both directions - down for west bound trains and up for east bound trains - an operating instruction is issued by the train controller to a locomotive engineer on a form called a Mis.51. This instruction tells the locomotive engineer where they can go to, what crossings they may have with other trains, where the crossing will be, any special instructions, and if automatic signalling is suspended between two locations due to faults. Instructions are also issued to track gangs who need to access the line for maintenance or inspections. 

Mis.51 Form used by a Locomotive Engineer or member of the infrastructure team.

Locomotive Engineers are also required to make a radio call prior to arriving at crossing station or a station at the boundary of the automatic signalling. This is so other trains or track gangs in the vicinity can hear what train number it is, where it is and the instructions it has from train control.

Crossings in single line automatic signalling terriorty takes time, so KiwiRail has started work to upgrade signals and install point motors at some stations. This work has been completed at Darfield with has started at Jacksons, Rotomanu, Moana and Kokiri. Operation of the points and signals will be controlled by train control so crossing will be significantly more efficient and Locomotive Engineers can remain warm and dry in their cabs.

Masts for new departure signals at the down end of Kokiri station.

For a railway line with a good number of trains, it's surprising that a system like single line automatic signalling remains in use and hasn't been replaced by more common forms of train control like centralised traffic control or track warrant control. KiwiRail have some very good reasons to keep the single line automatic signalling system in use. The existing system has sufficient traffic capacity for the 10 to 14 trains that use the Midland Line every day, geographical and weather challenges makes installing and maintaining fibre networks difficult, its proven reliability, and while there are signalling upgrades taking place at some crossing stations, the cost of installing a new CTC system across the whole Midland Line is prohibitive. 

This post gives a general overview of single line automatic signalling and definitely does not cover all the intricacies of this form of train control. 

If you want to find out more about how the signalling on the Midland Line works you can find KiwiRails Network Operating Instructions for the Midland Line here. A locomotive engineer that drives on the West Coast also often posts cabride videos of his trips on the Midland Line. You can see one of Stand Up West Coasts videos showing the trip between Stillwater and Moana with the automatic signalling here. 

And thats about it.