Railway Signalling

Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which makes it difficult to quickly stop when encountering an obstacle. From very early in its history, railway operators realised the need for a system of signalling to direct railway traffic and keep trains clear of each other at all times.

The earliest rail cars were hauled by horses or mules. A mounted flagman on a horse preceded some early trains. Hand and arm signals were used to direct the "train drivers". Foggy and poor-visibility conditions later gave rise to flags and lanterns. Wayside signalling dates back as far as 1832, and used elevated flags or balls that could be seen from afar.

The simplest form of operation, at least in terms of equipment, is to run the system according to a timetable. Every train crew understands and adheres to a fixed schedule. Trains may only run on each track section at a scheduled time, during which they have 'possession' and no other train may use the same section. When trains run in opposite directions on a single-track railway, meeting points ("meets") are scheduled, at which each train must wait for the other at a passing place. Neither train is permitted to move



With the advent of the telegraph in 1841, a more sophisticated system became possible because this provided a means whereby messages could be transmitted ahead of the trains. The telegraph allows the dissemination of any timetable changes, known as train orders. These allow the cancellation, rescheduling and addition of train services.

Most forms of train control involve movement authority being passed from those responsible for each section of a rail network (e.g. a signalman or stationmaster) to the train crew. The set of rules and the physical equipment used to accomplish this determine what is known as the method of working in the UK, method of operation in the United States or safeworking in Australia. Not all these methods require the use of physical signals, and some systems are specific to single track railways.

Block Signalling

Trains cannot collide with each other if they are not permitted to occupy the same section of track at the same time, so railway lines are divided into sections known as blocks. In normal circumstances, only one train is permitted in each block at a time. This principle forms the basis of most railway safety systems. Blocks can either be fixed (block limits are fixed along the line) or moving blocks (ends of blocks defined relative to moving trains).

Level Crossings

A level crossing is an intersection where a railway line crosses a road, path, or (in rare situations) airport runway, at the same level, as opposed to the railway line crossing over or under using an overpass or tunnel. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion.

Gated crossings became commonplace in many areas, as they protected the railway from people trespassing and livestock, and they protected the users of the crossing when closed by the signalman/gateman. In the second quarter of the 20th century, manual or electrical closable gates that barricaded the roadway started to be introduced, intended to be a complete barrier against intrusion of any road traffic onto the railway. Automatic crossings are now commonplace in some countries as motor vehicles replaced horse-drawn vehicles and the need for animal protection diminished with time. Full, half or no barrier crossings superseded gated crossings, although crossings of older types can still be found in places. In rural regions with sparse traffic, the least expensive type of level crossing to operate is one without flagmen or gates, with only a warning sign posted.


Photo: Stations Past Website

Signal Boxes

Back in the steam era, just about every station had a signal box which controlled all movements into and out of the station area, before a train can leave one station it has to be accepted by the Signaller at the next station. Signal boxes are shelters or small buildings constructed specifically to house the levers and other control equipment used to safely space, route and locally coordinate railway trains; and to provide signalling staff with a vantage point from which to safely observe and efficiently control nearby trains.

Australian Railway Signalling

Australian railway signalling varies between the states of Australia, because each railway system was established under the different colonial governments with separate legislation. Just as with the notorious variation of Australian rail gauges, there are differing signal systems. The systems in New South Wales generally follow British precedents, although American influence has increased somewhat since the 1990s. Victoria uses American-style speed signalling on its main lines. That can cause confusion where the systems meet.

For example, in New South Wales a green-over-red coloured light signal means "caution", indicating the next signal is at "stop". In Victoria, that same aspect means "clear normal speed", indicating the next signal is anything but at "stop". On the Main South Line from Sydney, single-light signals are now exclusively used from Spring Creek bridge (south of Galong) to Albury on the Victorian border. That forms a buffer zone between the conflicting signal indications of each state. Gradually, the remaining upper quadrant signals (and double-light signals at Binalong) are being replaced by single-light signals.



South Australia uses two primary forms of signalling. Nearly all signal boxes in South Australia have now been closed, and most rail traffic is coordinated through centralised traffic control (CTC) systems, either under the Australian Rail Track Corporation (ARTC) from Mile End or Adelaide Metro control from Adelaide. Where the two networks interface, such as at the Goodwood level crossing or at Torrens Junction, control is usually by the ARTC after release from Adelaide Metro.

The Western Australian system is simple to understand, and the complexities of leading position lights and other odd attachments to signalling apparatus have been either eradicated or were never part of it. Switchlocks are used to enter sidings from mainline CTC territory, such as the CBH Group yard terminal on the standard gauge railway. The train controller must release the switchlock, thus interlocking the signals to stop, which is similar to the system in Queensland. Perth's urban passenger network is operated by the Public Transport Authority and the rules are almost identical to those of freightlines.