What triggers the signals to change to danger (red)

[Ditop23]

Hi
When a train proceeds though a green, it will then reach the overlap point, which is about 200 yards aways from that signal. When the front wheels go over the overlap the signal changes to red. When the back wheels go over the overlap the previous signal before this signal changes from red to yellow.

Is there a sensor on the track or a transmitter that the train goes over at the overlap, to send the message to the signals to change?

Thanks

 

[dk1]

Hi
When a train proceeds though a green, it will then reach the overlap point, which is about 200 yards aways from that signal. When the front wheels go over the overlap the signal changes to red. When the back wheels go over the overlap the previous signal before this signal changes from red to yellow.

Is there a sensor on the track or a transmitter that the train goes over at the overlap, to send the message to the signals to change?

Thanks

Normally track circuits.

 

[Falcon1200]

I was a railway operator, not engineer, but my understanding is that in Track Circuit Block areas, signals are usually either first or last wheel replacement; ie the signal will revert to red when the front or the rear of the train passes it.

 

[Gloster]

A signal will go to red either when the front of the train is only a few yards past it, just enough to prevent the driver see it change, (first wheel replacement) or when the front is a couple of hundred yards beyond (as explained above). The previous signal will stay at red until the whole of the train has passed a fair way, usually a couple of hundred yards, beyond the signal. The principle is that if the driver of a second train slightly misjudges his stop at the signal, he won’t run into the back of the previous train which has stopped with the rear of the train immediately beyond the signal: this is known as the overlap.

I will leave others more technically gifted than me to explain track circuits and modern alternatives that register the position of the train.

 

[Ditop23]

Normally track circuits.

Thanks. To clarify, the signal is actually before the start of new section as apposed to the signal being the start of a new section.

 

[MarkyT]

Track circuits were used traditionally to prove the entire section and overlap ahead clear before allowing a signal to show proceed (yellow or better) and to automatically replace it to red when passed as described by others above.. They're increasingly replaced by axle counters in modern renewal schemes.

 

[Gloster]

Thanks. To clarify, the signal is actually before the start of new section as apposed to the signal being the start of a new section.

Not quite. The signal section runs from signal to signal, but as a typical piece of old railway ‘belt and braces’ practice, they allow a bit of an overlap just in case the driver of a second train slightly misjudged his stop and passed the signal. If train B can’t pass Signal 1 until the the previous train (train A) has passed Signal 2 and gone a couple of hundred yards, the driver of B has a slight margin if he should misjudge and stop a few yards past Signal 2, even if train A was still on its way to Signal 3. If there was no overlap train A could have stopped with its last vehicle immediately beyond Signal 2 and a slight misjudgement could mean that train B could hit it. (Of course, the driver shouldn’t make such a mistake, but railway rules and practices developed with the understanding that people do make mistakes and it was best to make allowances for that.)

 

[MarkyT]

Thanks. To clarify, the signal is actually before the start of new section as apposed to the signal being the start of a new section.

Many plain line signals have a combined berth and overlap arrangement, without a track circuit boundary or axle counter sensor near the signal. Many were known as automatic signals in the old days, but are not specially plated as such in new schemes. Where there are points in the route, or where reversing takes place routinely, there must be a track circuit/axle counter boundary at the signal giving authorisation.

 

[Ditop23]

Track circuits were used traditionally to prove the entire section and overlap ahead clear before allowing a signal to show proceed (yellow or better) and to automatically replace it to red when passed as described by others above.. They're increasingly replaced by axle counters in modern renewal schemes.

I may have worded my question poorly.
I was asking how the signal knows that it has to change when the train goes over the overlap point (200yards after the signal). Someone above said it's because the overlap point is the start of the next track section.

My new question is now,
Does this mean a signal is not the start of the next track section, (as i always thought).
The start of the next track section is actually the overlap. (which is about 200yards after the signal)??

Thanks

 

[dk1]

Thanks. To clarify, the signal is actually before the start of new section as apposed to the signal being the start of a new section.

It’s just before. Many platform signals stay as a proceed aspect until the entire train has passed. This is due to rules where the guard dispatches the train and should not see a red.

 

[MarkyT]

It’s just before. Many platform signals stay as a proceed aspect until the entire train has passed. This is due to rules where the guard dispatches the train and should not see a red.

Early colour light installations from the steam era often had delayed replacement, often by the second track going occupied rather than the first, to avoid the footplate crew in the typical cab position seeing a red. Last wheel, when the entire train has to pass the signal before returning to red, was also used for propelling shunt moves where the driver is right at the back of the train.

 

[dk1]

Early colour light installations from the steam era often had delayed replacement, often by the second track going occupied rather than the first, to avoid the footplate crew in the typical cab position seeing a red. Last wheel, when the entire train has to pass the signal before returning to red, was also used for propelling shunt moves where the driver is right at the back of the train.

Cheers.

 

[Ditop23]

Early colour light installations from the steam era often had delayed replacement, often by the second track going occupied rather than the first, to avoid the footplate crew in the typical cab position seeing a red. Last wheel, when the entire train has to pass the signal before returning to red, was also used for propelling shunt moves where the driver is right at the back of the train.

Is there a sensor on the track which sends a message for the signal to change?

 

[alxndr]

Is there a sensor on the track which sends a message for the signal to change?

No, the track circuit is the “sensor”.

In its simplest form* a track circuit is an electrical circuit which uses the rails like wires. The track is divided into sections with Insulated Block Joints, and a power supply is connected to one end of this. The other end is connected to a relay. When there is no train on the track circuit the current will reach the relay, and so relay will energise. When there is a train the wheels and axle will form a short circuit across the rails, and no current will reach the relay, so it de-energises.

The interlocking, which will either be relay based or use solid state electronics, will determine the aspect of the signal based on the state of the relay (and any other applicable factors).

*Track circuits can be more complex to avoid the need for IBJs, or you can use axle counters instead, but this is the basics.

 

[Gloster]

Just to add to the above that when there is a train the current goes across to the other rail through the wheels and axles. It is each individual rail, one under one side of the train and the other under the other, that form the two parts: a pair of wheels will have one on each rail. The rails are insulated from the sleepers and everything that holds them in place and from the next and previous ones.

 

[Steve Harris]

It might be a good idea to have a look at https://www.railwaysignalling.eu/wp...track-circuits-detect-and-protect-trains1.pdf which explains things in detail.

Or just ask Google (as the above link + 10 AI answers were thrown up after a quick search) which will probably explain what you want to know.

 

[norbitonflyer]

Is there a sensor on the track which sends a message for the signal to change?

Essentially yes - the track circuit is the sensor. The wheels and axles complete a circuit through the running rails, operating a relay switch which, among other things, causes the signal controlling that section of track to switch to a red aspect.

 

[Phil from Mon]

To follow (logically?) from this, and as they have already been mentioned, how do axle counters work, given that trains have differing numbers of axles? Is it something programmed from the train description (eg 1M65)?

 

[MarkyT]

To follow (logically?) from this, and as they have already been mentioned, how do axle counters work, given that trains have differing numbers of axles? Is it something programmed from the train description (eg 1M65)?

There are inductive wheel sensors at each extremity of a train detection section. The sensors register each wheel passing and its direction. An evaluator unit keeps track of all these events, counting axles into the section, then counting them out again. Only once the evaluator is satisfied that there are zero axles within the section will it output a clear state to the interlocking.

 

[Mcr Warrior]

Always slightly disconcerting, when sat towards the back of a lengthy train set, such as the 11 car Avanti units on the WCML, that every set of signals you pass by, is showing a red aspect!

 

[AF91]

Just to complicate matters, some signal sections now use axle counters rather than track circuits. There is a sensor at the start of the section that counts the number of wheels that enter and a sensor at the end of the section that counts the wheels back out again. If the exit sensor counts fewer wheels out the other end the signaling system knows that part of the train has been detached.

 

[Haywain]

Always slightly disconcerting, when sat towards the back of a lengthy train set, such as the 11 car Avanti units on the WCML, that every set of signals you pass by, is showing a red aspect!

I would only find that disconcerting if it was accompanied by a very heavy brake application!

 

[MarkyT]

Just to complicate matters, some signal sections now use axle counters rather than track circuits. There is a sensor at the start of the section that counts the number of wheels that enter and a sensor at the end of the section that counts the wheels back out again. If the exit sensor counts fewer wheels out the other end the signaling system knows that part of the train has been detached.

Axle counters are the default for new schemes, whether colour light signalling or ETCS. The UK started using axle counters in the 1980s, but they were considered expensive and rather novel at the time, despite established widespread use in mainland Europe, particularly Germany. On the other hand, they could save money for particularly long block sections because each individual conventional track circuit is technically limited to about a mile in length so to cover a ten mile long section you would need ~10x track circuits, all logically summed in the block occupancy and requiring multiple cabinets, cabling and power distributed along the trackside. With axle counters, much of this can be avoided. Sensors are only necessary at the logical block extremities with a data comms link from each to an evaluator site. Early UK installations also included sites where tradtional track circuits had proven unreliable due to difficult environmental conditions, notably salt water or other contamination. Examples include the Forth Bridge, the Severn Tunnel and the Dawlish sea wall.

 

[norbitonflyer]

To follow (logically?) from this, and as they have already been mentioned, how do axle counters work, given that trains have differing numbers of axles? Is it something programmed from the train description (eg 1M65)?

There are two sensors at the entrance and exit of the section, respectively counting how many axles have entered the section, and how many have left it. The signal will not clear unless those two numbers are equal. It doesn't matter how long the train is - it may have three axles (eg a class 08 running light engine) or 200 (eg a train of 47 bogie wagons hauled by two class 66s)

 

[MarkyT]

There are two sensors at the entrance and exit of the section, respectively counting how many axles have entered the section, and how many have left it. The signal will not clear unless those two numbers are equal. It doesn't matter how long the train is - it may have three axles (eg a class 08 running light engine) or 200 (eg a train of 47 bogie wagons hauled by two class 66s)

Some early axle counters had a maximum count of 256, which was a problem for long freights even in the UK. More modern systems don't have this constraint, or more likely the maximum is so high it is not a practical limitation for operations even in the USA.

 

[Ditop23]

Thanks all.

Just to confirm with ref to the TCB.

When the Front wheels go past the signal, it instantly changes that signal to red.

When the Back wheels go over, the signal before changes to yellow.

My question is,
Are these at 2 different locations on the track? Is the first point at the start of the overlap and then 2nd point at the end of the overlap?

Thanks

 

[Gloster]

Thanks all.

Just to confirm with ref to the TCB.

When the Front wheels go past the signal, it instantly changes that signal to red.

When the Back wheels go over, the signal before changes to yellow.

My question is,
Are these at 2 different locations on the track? Is the first point at the start of the overlap and then 2nd point at the end of the overlap?

Thanks

No. In some locations the signal (call it 1) goes to red as soon as the front of the train passes it, in others the train will have to proceed a short distance (no more than a couple of hundred yards at most) before the signal goes to red. The choice of which arrangement is used depends on the particular situation. 1 will remain at red until all the train has passed signal 2 and gone a distance, again probably a few hundred yards, beyond: only then will 1 go to yellow. This is the overlap in case the next driver approaching 2 misjudges his braking and goes a bit past it: if there wasn’t an overlap then the back of the first train could be immediately beyond signal 2.

 

[Annetts key]

Okay, lots of replies here.

But some clarification is needed. I talk about track circuits here, but they can be axle counter sections instead.

To keep things simple, the following refers to track circuits as if they are all single track circuit sections. In practice, they may actually be made up of more than one track circuit section, but the interlocking combines them and treats these groups as if they are a single track circuit section.

Hence there may be the following track circuits along a line in sequence: AA/1, AA/2, AA/3, AA/4. The interlocking combines the outputs from them to operate a 'repeat' relay, which may be called AA T2PR. It is this summation relay that the interlocking uses, hence the interlocking treats AA/1, AA/2, AA/3, AA/4 as one track circuit section.

Now, the signals...

First, I need to tell you that there are controlled signals and automatic signals.

A controlled signal normally will only clear when the signaller operates his/her control for it. If the interlocking determines that the section is clear of trains, the next signal is lit and any other requirements are met, the Interlocking will clear the signal to an appropriate aspect. Once a train has been detected as having passed the signal, it will go back to red and stay at red. It will not normally reclear without the signaller re-operating the control for it.

An automatic signal normally works automatically with no intervention from the signaller. They are normally used on plain line where there are no points or level crossings (that use signals for protection).
If the section ahead is detected clear of trains, the next signal is lit, and any other conditions are okay, the signal will clear to an appropriate proceed aspect. It will go back to red once a train passes it (* but see later) but once the section is clear again, it will automatically reclear.

For a normal controlled signal, there is a track circuit all the way up to the signal (often called the berth track). Then you have the signal. Then there is an overlap track circuit of between 50 yards and 300 yards. Normally an overlap is at least 200 yards. This is often called the overlap track. Then there is the next track circuit up to the next signal (the berth track for it).

It is the interlocking detecting the overlap track being occupied by a train that causes the signal to go back to red. Although note that any track circuit going occupied between the two signals will cause the signal to go to red.

Most automatic signals don't have a separate overlap track circuit. Instead the berth track circuit continues past the signal and ends at the end of the overlap (in other words at least 200 yards beyond the signal). It is the next track circuit that causes the signal to go back to red. Typically the berth track for the next signal. This does mean that if you are a passenger near the front of the train, you may see the signal still showing a proceed aspect out the window, well, until the front axle goes beyond the overlap and on to the next track circuit.

One of the other posters talked about last wheel replacement. These are not very common. They are controlled signals with special controls. Typically only found on platform lines at stations or depot lines or similar where shunting or propelling moves may be made. As they have said, the signal does not go back to red when a train goes past it. When it goes to red depends on the requirements at the location where each particular signal is located. It may also depend on the length of the train.

 

[edwin_m]

To put it as simply as possible, for signal 1 to clear from red, all the track circuits or axle counters as far as the end of the overlap of the next signal (signal 2) must confirming that there is no train present. There is not really a message sent to the signal to tell it to change, there is some logic (in electrical relays or software) that is continuously monitoring these detectors (plus some other things not relevant here) and continuously telling the signal what aspect it should show.

The complication, as mentioned above, is that for most signals that only have plain line up to the end of the overlap (so no points, level crossings etc), the detection section leading up to the signal continues to the end of its overlap. If signal 1 is one of these, then it won't go back to red until the front of the train passes the end of the overlap of signal 1, but the previous signal (signal 0) will stay red until the rear of the train has passed the same place. So there will always be at least one red signal behind the train.

I'm ignoring last wheel replacement, subsidiary aspects and other complications.

 

[Phil from Mon]

Great explanations, thanks very much