NZR Signals

Introduction

Signalling of trains became a must very soon after their acceptance as a viable means of transport. All early developments were in Britain where a host of different systems were developed, some surviving and developed further into more modern signally systems, some only used on an individual line and soon replaced by what were seen as better systems. The need for signals was of course an attempt to prevent the spate of horrific accidents that occurred, and as no system eliminated all types of accidents, the things that were learned from each subsequent accident, helped define new goals for the systems that were in use.

There are several aspects to a signalling system, the physical signals can indicate to a train driver that he has permission to proceed. In the early days of railways that was all that was provided and the driver could assume that there was no other train on the track. Where a train's route was long, signals could be positioned at locations along that route to keep trains at a safe distance from the train ahead.

As the railway became more complex, particularly with regard to station yards, the signalling system also had to ensure that the way was safe for the train to travel through the many sets of points. This meant a physical connection was required between those points (turnouts) and the signal box, so the signalman could set a safe route. Such a system was called interlocked, in that, provided the signalman had done his job properly, it 'locked' the route for the expected train.

All early signalling systems required a signalman to control the individual signals and the set of points that it was usually associated with. At stations there were often too many signals for one signal man to work because they were not always close together, and so the concept of the signal box evolved, where one signal man could control many signals. There was a physical connection between a lever in the signal box and a signal or the movable points of a turnout. The signals were usually connected by a wire cable that was held under tension with a counterweight at the signal, but the points were connected by steel rods, because the action had to be able to both push and pull to effect the required change. For very big yards, two or even three signal boxes were were required as there was a limit to how far the mechanical links could operate reliably.

The New Zealand Scene

New Zealand was in some ways in a fortunate position, as the introduction of railways here was still a good number of years behind developments in Britain. This both allowed us to learn from some of the early mistakes made there, and also be in a position to purchase ready made solutions to the challenges that were faced in the world of railway signalling. The early railways systems in New Zealand were so independent of each other that it is a bit surprising that the basics of their safety systems were still quite similar. The operation of trains invariably requires a set of rules to allow that to happen safely, so those early systems were led by Canterbury, where operations began in 1863 but it was not until 1866 that they adopted their first rule book. It can only be assumed that train frequency and the limited track meant only one train was running at any one time. Southland's early railway ambitions failed and again their limited trackage and low train frequency probably meant they did not yet require formal rules. So it was snot until 1872 that Otago produced a set of railway operating rules. Both Canterbury and Otago introduced semaphore signals at defined locations to indicate to train drivers the status of the way ahead.

Once Sections of track that had been constructed under the Railways Act 1872 became operational, they were operated under an NZR set of rules that came into force on 10 September 1874

Signals are the interface between the tracks that trains run on, and the drivers of the trains that need to run. As such they need to operate in a reliable way and all systems that are installed are therefore subject to regular testing to ensure they meet the required standards. Signalling systems are inherently complex, and standardising the way they behave, ensures they do so in a predictable way. A train driver can then confidently operate his train on completely different sections if they both have the same signalling system. There is a further interface between the signalling system and the driver, and that is known as the Signalling and Interlocking (S&I for short) Circular (or Diagram). These documents record the tracks and signals on the various section of a railway and their relative positions. The train driver needs to be familiar with the circulars applying to his route and will invariably have to refer to the relevant one if something out of the ordinary happens. These diagrams started to be produced once interlocking systems were developed and were progressively produced as the various station had better signally installed.

The simplest way to introduce the signalling systems used in New Zealand is to first get an overview of the systems used, and then to look at how those systems were adopted on the various line in the New Zealand network.

The systems of working trains

The type and placing of signals was intimately associated with the rules that Railway staff had to abide by, so the Rules defined how a Driver had to respond to any signals he might encounter. A train driver therefore had to have a thorough knowledge of those rules. The introduction of new rules was usually triggered by some sort of mishap, not necessarily in New Zealand, that highlighted the risk a particular situation might present. This invariably got creative minds working on the problem and new equipment was invented to help prevent a recurrence of the original trigger. In this way it can be seen that signalling systems developed over a significant period of time, and different sections of a network as spread out as it was in new Zealand would end up with different systems operating on different line, all based on the risk associated with the operations on an individul line.

Block Working

The safest way to operate a railway is to only have one train working at a time, and many portions of our railways both started and ended that way. The Greytown Branch for instance, was only about 5km long, and its whole life was operated by an engine(s) based at Greytown. With a maximum of five trains each way a day, safe operation was very straightforward. As soon as line started to get longer, there was the need to have more than one train operating on the mainline at any one time, and for that to be safe, there were two situations that had to be prevented, a) trains traveling on opposite directions on the same track could collide, or b) one train following another on the same track could run into the back of it. The basic principal adopted to prevent those two situations occurring was what is termed "Block Working". This allowed only one train to be in a block (or section of track) at any one time. However, it is one thing to state that that is how trains will operate, it is quite another to implement it in a practical way. Many systems have been invented to attempt to guarantee only one train could be in any one block at a time, but no system is fail-safe where human operators are involved. Things go fine if everyone obeys the rules, but if someone is intent on breaking them or inadvertently forgets something, thinks can still go horribly wrong.