British Mechanical Signalling

"Pure" non continuous signalling

- Including many of the british ex-colonies, such as India, Pakistan, parts of Australia etc
- Some cases of Portuguese mechanical signalling

In this system, no signal indicates the aspect of the following signal, therefore, you do not have the denominated "aspect continuity". There is a distant signal per signal post wich only will be open if all stop signals on the train path operated from that signal post are open AND that path is the one considered the "direct" path.

If any of the stop signals is closed or a diverging path is set, the distant will be closed. As there is no distinction between the case where any of the stop signals is closed and where a diverging path is set the driver will obviously consider that the first stop signal in it's way is closed. the speed points are to be known mentally by the driver, to whom precise direction indictations will be given by the signalling.

Basically a block is either occupied or free in the drivers point of view, as even if the signal at the junction gives him a diverging path he will not know the aspect of the "exit" starter wich normally exists after the junction (we don't want to hold a train before a junction, therefore blocking it, do we!!!!).

There are however "subjective" means to know that the following signals are open, UK signalman, when any of the stop signals of that S'Box is closed, are instructed to only open the previous stop signal when the train is preparing to stop at it. Therefore if the driver approaches the stop signal already open after a closed distant he might assume with some degree of confidence that all the following stop signals of that S'box are open.

For more info go to:
http://www.trainweb.org/railwaytechnical/sig-uktypes.htm

French Mechanical Signalling

Continuous signalling within the station limits, non continuous between stations

- Some cases of Portuguese mechanical signalling (Portuguese signalling has strong British and French signalling)
- Portuguese Interposts Block (Aut Sig)
- Some cases of the French BAPR (Aut Sig)

In these systems, the exit signal of the station (or in US terminology "interlocking") is only equipped to present the aspects "stop" or "all clear" at the next signal. Therefore, the next station's entrance signal aspect will only be indicated at the braking distance of it, by means of a distant signal (advanced signal under Portuguese signalling) wich will never present a stop aspect, therefore not serving as "protection" to keep two trains apart.

This distant, unlike the british distant will only refer to the entrance signal. If the exit signal is closed, the home signal will present caution (= "get ready to stop" at the next signal) but the distant might also present caution as the entrance signal might be very close to the exit signal. An average european station is no more than 500 meters long and a train at 70 mph doesn't stop in 700 meters.

The entrance signal usually allows some space for shunting or in the case of automatic signalling, the so called "caution warning" wich usually means get ready to stop at the second signal.

For more info go to:
http://www.zonaweb.pt/hjcl

Portuguese Automatic Block with Advanced Signals

Continuous signalling within the station limits, non continuous between stations but with multiple blocks between stations - ALLWAYS REQUIRES AUTOMATIC SIGNALLING

- Some cases of the French BAPR

The principal is precisely the same as the one given above, HOWEVER, at least two blocks exist between two stations, therefore requiring at least a main line stop signal (number of blocks, less one) and a distant to each one of the stop signals. The stop signals will only show stop or all clear while the distants will only show caution or all clear, in accordance to it's respective stop signal.

North American Automatic Signalling

Continuous signalling - ALLWAYS REQUIRES AUTOMATIC SIGNALLING

- French BAL (Luminous Automatic Block)
- Portuguese Pure Automatic Block

In this case, each signal aspect is dependent of the next signal aspect. It's the basic familiar principal, "after the yellow comes red" etc etc etc.

Unusual Continuous signalling

- German H/V signals system
- COMECOM signalling - Old East Germany, eastern Block, Russia and China - the old Eastern block managed to standardise railway signalling. Signal heads might change but not the basic aspects which means about the same thing.

In these countries, each stop signal has it's own distant, wich might be together with the previous stop signal if the stop signals are close together; or at the braking distance of it's respective stop signal. In my opinion this is the best one around despite some inconveniences.

For more info go to:
http://home.t-online.de/home/wmeyenberg/eisenbahn/

Signal States

The Fundamental orders are:

- Stop
- Caution
- All Clear

The caution will need to be associated with a reference speed, usually 20mph (30 or 40 kph) to be complied over reversed points. If you need to slow down a train because of points, there is a Caution at the signal before the points, being it preceeded in it's turn by a Caution also. This is the british principle although it does not appear so. Simply think that the value of a stop signal cleared is given by it's distant. If the distant was closed, then the cleared stop signals at that station will stand for caution. If the distant was cleared, then all the signals will stand for all clear. The last signal controlled by a S'box will allways stand for all clear as it lets the train go into another block.

If you want to increase speeds, and/or shorter blocks and avoiding using the caution followed by caution, the following is required:

- Stop
- Caution
- Caution Warning
- All Clear

The caution will need to be associated with a reference speed, usually 20mph (30 or 40 kph) to be complied over reversed points.

If besides that, you want to inform better the drivers about the reason why they must slow down, which in the signalling point of view might be only one of two reasons, SPACING OR POINTS you'll need:

- Stop
- Caution
- Caution Warning
- All Clear
- Diverging path
- Warning of Diverging Path

The caution will NOT need to be associated with a reference speed, to be complied over reversed points as there are specific aspects to that. The Diverging path and Warning of Diverging Path aspects, can either be integrated with the other aspects (Portugal and the Ks system, in Germany) or be independent of them like in France; where these aspects, wich exist for 30 and 60 km/h, are shown individually and the block aspects, caution and caution warning are also shown if required (the all clear is not shown).

Considering this, railways can choose either to use a range of aspects each one referring to a speed or then use a single aspect standing for diverging path and refer to a numerical indicator indicating the speed divided by 10; i.e.6 at the indicator will stand for 60.

The same thing applies for the warning, but the warning must be given in almost the same way as the "execute" aspect; as the driver must be able to easily associate the warning with the execute aspect.

Further, if trains run over 100 mph (160 km/h) you'll need an additional aspect to comand that a train slows down to that speed; usually Flashing green (in Europe). Without that, no trains can operate over 160 km/h. In Portugal, things are different, as usual, instead of 160 it's 140 and it's flashing green that allows trains to run over 140, as long the ATP is working!.

Another option is using the "direction indication". The need for this will vary according to the signalling aspects used and the "spirit" of the regulations, wich obviously varies from country to country.

While in the UK, it is the whole base of the signalling being given all the times, in Belgium and in the US it is supressed, as you only know the you are going to a deviated track. Here in Portugal, when necessary, it is given in color light signals and rarely on mechanical signals, but then most Portuguese stations signalled with mechanical signals only have distant signals. This is quite an efficient system despite what it seems, allowing quite good average speeds.

Special Aspects

Permission to enter into an occupied block - "Permissive Stop" This permission might be Permanent, in the case of block signals wich don't protect points or blocks wich don't need to be Absolute. A train must temporarily stay at the signal wich protects the block when for example, a station is closed and the running paths are permanentely set or sporadic; or when for some reason a train needs to enter into an occupied line.

A train might or not be required to stop at the signal wich protects the occupied block. The general rule is that a stop and go must allways be done but there are cases where a train is allowed into an occupied block without stopping, such as on a grade, where heavy trains might have difficulty in restarting if they stop and you will want to avoid their stop unless absolutely necessary.

Usually automatic signals are equipped with broken rail detection (as long track circuits are used) and if a rail is broken, the preceding signal to the rail closes. For this reason, after passing a closed signal, the driver must proceed ready to stop short of any obstruction until the tail of the train passes the next signal.

Shunt Aspects - In Europe, when ever a train is to make movements inside a station, there are smaller signals, wich are ignored by scheduled services wich apply only to station movements. These signals, basically divide station blocks into smaller blocks and more easy to handle as such movements are made at a low speed, say 20 mph, and so signalling can be more flexible and movements faster.

If you want a good signalling use something like the German Ks or the new Swiss system, wich are simple systems, using speed indications associated to color light aspect. For you it would look like this:

- Fixed Red - Stop
- Fixed Red with Lunar White light - Permissive stop
- Flashing Red - Proceed ready to stop short of any obstruction
- Yellow - Caution, next signal closed
- Flashing Yellow - Caution Warning, 2nd signal closed OR short braking distance between the next signal and the 2nd signal wich protects a diverging path.
- Flashing Green - All clear, speed indicated
- Green - All Clear, line speed permissable

speeds would be indicated in this way:

- The speed to be complied at the next signal, would be shown above the signal head in yellow numbers divided by ten, that is 100 would show 10
- The speed to be complied at that signal would be shown belowthe signal head in white numbers divided by ten, that is 100 would show 10
- Speeds would never be shown with the "red" permissive aspects
- All the other aspects would be shown normally independentely of the speed indicated, except the green which would be shown flashing if a speed was associated to it, and fixed if no speed associated with.
- The indication of speeds denote ALLWAYS a diverging path.
- Regarding to direction indications, they would be useful but not necessary.

For more info go to:
http://www.trainweb.org/railwaytechnical/sigtxt5.html
http://www.trainweb.org/railwaytechnical/sigtxt1.html

Written by Hugo Leandro