Signalling used on high density metro (or subway) routes is based on the same principles as main line signalling. The line is divided into blocks and each block is protected by a signal but, for metros, the blocks are shorter so that the number of trains using the line can be increased. Originally, metro signalling was based on the simple 2-aspect (red/green) system as shown above (click for full size view). Speeds are not high so three-aspect signals were not necessary and yellow signals were only put in as repeaters where sighting was restricted.
Many metro routes are in tunnels and it has long been the practice of some operators to provide a form of enforcement of signal observation by installing additional equipment. This became known as automatic train protection (ATP). It can be either mechanical or electronic.
The London Underground, for example, uses both types on its lines, depending on the age of the installation. The older, mechanical version is the train stop, the electronic version depends on the manufacturer. The trainstop consists of a steel arm mounted alongside the track and which is linked to the signal. If the signal shows a green or proceed aspect, the trainstop is lowered and the train can pass freely. If the signal is red the trainstop is raised and, if the train attempts to pass it, the arm strikes a "tripcock" on the train, applying the brakes and preventing motoring.
Electronic ATP involves track to train transmission of signal aspects and (sometimes) their associated speed limits. On-board equipment will check the train's actual speed against the allowed speed and will slow or stop the train if any section is entered at more than the allowed speed.
The Overlap
If a line is equipped with a simple ATP which automatically stops a train if it passes a red signal, it will not prevent a collision with a train in front if this train is standing immediately beyond the signal. There must be room for the train to brake to a stop. This is known as a "safe braking distance" and space is provided beyond each signal to accommodate it. In reality, the signal is placed in rear of the entrance to the block and the distance between it and the block is called the "overlap". Signal overlaps are calculated to allow for the safe braking distance of the trains using this route. Of course, lengths vary according to the site; gradient, maximum train speed and train brake capacity are all used in the calculation.
This diagram (left, click to enlarge) shows the arrangement of signals on a metro where signals are equipped with trainstops (mechanical ATP) and each signal has an overlap whose length is calculated on the safe braking distance for that location. Signals are placed a safe braking distance in advance of the entrances to blocks. Signal A2 shows the condition of Block A2, which is occupied by Train 1. If Train 2 was to overrun Signal A2, the raised trainstop (shown here as a "T" at the base of the signal) would trip its emergency brake and bring it to a stand within the overlap of Signal A2.
Overlaps are often provided on main line railways too. In the UK, it is the practice to provide a 200 yard (185 m) overlap beyond each main line signal in a colour light installation. In the US, the overlap is considered so important that a whole block is provided as the overlap. We will see more about this in Automatic Train Protection below.
Track-Circuited Overlaps
Nothing in life is as simple as it seems and so it is with overlaps. A line which uses overlaps and has close headways could have a situation as shown here (click for full size view) where the train in the overlap of Signal A1 has a green signal showing behind it. Although it is protected by Signal A2 showing red, the driver of Train 2 may see the green signal A1 behind Train 1 and could "read through" or be confused under the "stop and proceed" rule.
So, where there is a possibility of a green signal being visible behind a train, overlaps are track circuited as shown left. Although there is no train occupying the block protected by Signal A1, the signal is showing a red aspect because the train is occupying the overlap track circuit. This will give rise to two red signals showing behind a train whilst the train is in the overlap.
Automatic Train Protection
To adapt metro signalling to modern, electronic ATP, the overlaps are incorporated into the block system. This is done by counting the block behind an occupied block as the overlap. Thus, in a full, fixed block ATP system, there will be two red signals and an unoccupied, or overlap block between trains to provide the full safe braking distance, as shown here (click for full size view). As an aside, remember that, although I have shown signals here, many ATP equipped systems do not have visible lineside signals because the signal indications are transmitted directly to the driver's cab console (cab signalling).
On a line equipped with ATP as shown above, each block carries an electronic speed code on top of its track circuit. If the train tries to enter a zero speed block or an occupied block, or if it enters a section at a speed higher than that authorised by the code, the on-board electronics will cause an emergency brake application. This is the system used by London Underground for the Victoria Line from 1968 - the first fully automatic, passenger carrying railway ( more information here). It was a simple system with only three speed codes - normal, caution and stop. Many systems built since are based on it but improvements have been added.
ATP Speed Codes
A train on a line with a modern version of ATP needs two pieces of information about the state of the line ahead - what speed can it do in this block and what speed must it be doing by the time it enters the next block. This speed data is picked up by antennae on the train. The data is coded by the electronic equipment controlling the track circuitry and transmitted from the rails. The code data consists of two parts, the authorised speed code for this block and the target speed code for the next block. The diagram below shows how this works.
In this example (left), a train in Block A5 approaching Signal A4 will receive a 40 over 40 code (40/40) to indicate a permitted speed of 40 km/h in this block and a target speed of 40 km/h for the next. This is the normal speed data. However, when it enters Block A4, the code will change to 40/25 because the target speed must be 25 km/h when the train enters the next Block A3. When the train enters Block A3, the code changes again to 25/0 because the next block (A2) is the overlap block and is forbidden territory, so the speed must be zero by the time train reaches the end of Block A3. If the train attempts to enter Block A2, the on-board equipment will detect the zero speed code (0/0) and will cause an emergency brake application. As mentioned above, Block A2 is acting as the overlap or safe braking distance behind the train occupying Block A1.
Operating with ATP
Trains operating over a line equipped with ATP can be manually or automatically driven. To allow manual driving, the ATP codes are displayed to the driver on a panel in his cab. In our example below, he would begin braking somewhere around the brake initiation point because he would see the 40/25 code on his display and would know, from his knowledge of the line, where he will have to stop. If signals are not provided, the signal positions will normally be indicated by trackside block marker boards to show drivers the entrances to blocks.
If the train is installed with automatic driving (ATO - Automatic Train Operation), brake initiation for the reduced target speed can be by either a track mounted electronic "patch" or "beacon" placed at the brake initiation point or, more simply, by the change in the coded track circuit. Both systems are used by different manufacturers but, in both, the train passes through a series of "speed steps" to the signalled stop.
When the first train clears Block A1, the codes in Blocks A2, A3 and A4 will change to the next speed up and any train passing through them will receive immediately a new permitted speed and a new target speed for the next block. This allows an instant response to changing conditions and helps to keep trains moving.
Distance-to-Go
The next stage of ATP development was an attempt to eliminate the space lost by the empty overlap block behind each train. If this could be eliminated, line capacity could be increased by up to 20%, depending on block lengths and line speed. In this diagram, the train in Block A1 causes a series of speed reduction steps behind it so that, if a following train enters Block A6, it will get a reduced target speed. As it continues towards the zero speed block A2, it gets a further target speed reduction at each new block until it stops at the end of Block A3. It will stop before entering Block A2, the overlap block. The braking curve is shown here in brown as the "standard" braking curve.
To remove the overlap section, it is simply a question of moving the braking curve forward by one block. The train will now be able to proceed a block closer (A5 instead of A6) to the occupied block, before it gets a target speed reduction. However, to get this close to the occupied block requires accurate and constant checking of the braking by the train, so an on-board computer calculates the braking curve required, based on the distance to go to the stopping point and using a line map contained in the computer's memory. The new curve is shown in blue in the diagram. A safety margin of 25 metres or so is allowed for error so that the train will always stop before it reaches the critical boundary between Blocks A2 and A1. Note that the braking curve should reduce (or "flare out") at the final stopping point in order to give the passengers a comfortable stop.
Speed Monitoring
Both the older, speed step method of electronic ATP and "distance-to-go" require the train speed to be monitored. In Fig 8 above, we can see the standard braking curve of the speed step system always remains inside the profile of the speed steps. The train's ATP equipment only monitors the train's speed against the permitted speed limit within that block. If the train goes above that speed, an emergency brake application will be invoked. The standard braking curve made by the train is not monitored.
For the distance-to-go system, the development of modern electronics has allowed the brake curve to be monitored continuously so that the speed steps become unnecessary. When it enters the first block with a speed restriction in the code, the train is also told how far ahead the stopping point is. The on-board computer knows where the train is now, using the line "map" embedded in its memory, and it calculates the required braking curve accordingly. As the train brakes, the computer checks the progress down the curve to check the train never goes outside it. To ensure that the wheel revolutions used to count the train's progression along the line have not drifted due to wear, skidding or sliding, the on-board map of the line is updated regularly during the trip by fixed, track-mounted beacons laid between the rails.
Operation with Distance-to-Go
Distance-to-go ATP has a number of advantages over the speed step system. As we have seen, it can increase line capacity but also it can reduce the number of track circuits required, since you don't need frequent changes of steps to keep adjusting the braking distance. The blocks are now just the spaces to be occupied by trains and are not used as overlaps as well. Distance-to-go can be used for manual driving or automatic operation.
Systems vary but often, several curves are provided for the train braking profile. This example shows three: One is the normal curve within which the train should brake, the second is a warning curve, which provides a warning to the driver (an audio-visual alarm or a service brake application depending on the system) and the third is the emergency curve which will force an emergency brake if the driver does not reduce speed to within the normal curve.
Why doesn't everyone use distance-to-go? Partly because the systems used by many operators were installed before distance-to-go became available. Also, some operators require the safety margin, particularly in the US where they insist on an extra margin, known as the "lurch" factor, to allow for a train which decides to "motor" instead of "brake", as once happened in San Francisco.
