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In response to the talk comparing US and Australian practice, Victorian Railways at least use two main signalling systems. The first system is the Absolute Block System (ABS) and can be divided into sub-types such as Double Line Block. Basically this system works as previously stated in an above post where the concept is that the line is broken up into blocks where only one train may occupy a block at a given time. The signals used under the this system mostly only display red or green indicating that a train may either enter the block or it may not. That is the basic concept and there are other signals that apply in this system such as distant signals, which are similar to permissive in that their stop position isnt actually stop but means that the NEXT signal (or one of) is at stop. It is a bit more complicated but that is the basics. Turnout positions consist of two standard signals on a single bracket positioned to the left and right of each other. At no time will both signals display green (interlocked) and it works on the basis of left signal = green, train will take the left route. Right signal = green, train will take the right route. Even tho the two signal heads are next to each other on a horzontal plane, often you will find one positioned higher than the other indicating the 'main' or non-diverging route. This ABS system was (and is) used extensively on rural lines where traffic is of a medium density and blocks generally extend the entire length between towns or junctions. This system (at least the victorian version) required a lot of man power but was without a doubt the safest form of track protection for reasons i wont go into right now. The other primary system in Victoria is one i think you will find is similar to the US systems as i think that is what it was based on. This system is the Three Position System and the resounding difference between this and ABS (Victorian style) is that Three Position introduced the use of three signalling indications (red, amber or yellow, and green) as opposed to just red and green. This system was introduced i think about 1915 to better handle the congestion on the Melbourne Suburban system. It allowed trains to operate closer together because the signal system worked on speed indications and gave the drivers advance notice of what the next signal was indicating. IT consists mostly of a pair of signal heads mounted vertically on the same post. All targets are illuminated at the same time and thus indicate speed. The higher light indicates high (or 'normal') speed and the lower light medium or low speed. In some cases there many even be a third light for very low speed. I.E. RED over RED = STOP. RED over GREEN = medium speed CLEAR GREEN over RED = normal speed CLEAR YELLOW over RED = normal speed WARNING -> indicates that the driver should be prepared to stop. There are at least seven possible indications on the victorian system i believe. One interesting note that i believe is different to the US is that the arrangements of lights indicate whether the signal is an automatic type or manually controlled. If the lights are perfectly vertical over each other then the signal is manual and doesnt not show a permissive stop. When a train comes across a manually controlled signal that shows stop is must STOP and not move past the signal. A signal where the targets are offfset or 'staggered' above each other is defined as an automatic type that is not controlled by anyone except a track circuit (and other things like level crossings). An automatic signal displaying stop -> the train must come to a complete stop but may then proceed at a speed whereby the train may be able to pull up short of ANY obstruction. Similar to US practice. Although not strictly an indication of route it could be used as such by the drivers. For example a signal leading to a turnout. Providing the line was clear to begin with -> a RED over GREEN aspect would indicate MEDIUM SPEED therefore suggesting the turnout was lined for the diverging route. Or GREEN over RED would be a NORMAL SPEED aspect advising the driver the the turnout is aligned for the straight route. All in all i guess to the driver it doesnt matter where he is going as long as he knows what speed to go. One point is that ABS shows no speed indication. The system assumes that the drivers know the routes and the various speeds are posted on signs. In Melbourne on some signals (3 position system) there are true turn out indicators. These are either white illuminated arrows or feather systems. These can be seen at North Melbourne station at the entrance to the underground. The reason there are here (and in other places) is that the speed is the same no matter which route the train takes and at this particular station there are large numbers of trains that go either way so the turnouts are changing position every minute or two. One final note is that one some convential three position signals (particularly at Nth Melb) there signal has a display with a letter. This letter is a shorthand indication to the driver which way the route is lines up for a junction some kilometres down the track. "E" means he will be going to Essendon while "F" means he is going to Footscray. As far as i am aware this little feature is not an 'official' indication in that it should in no way affect the trains speed etc but is purely (i think) just to inform the driver the way route is lined up further along where he cant see yet. This is epecially true because departing north melbourne you have a choice of about four different routes all diverging within a couple of kilometres. Thats enought rambling for me now!! In my opinion the ABS system is simple to use and easy to install. Especially for many of us who arent signalling or electronics experts.
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