Implied signalling speed limits?

[karlbbb]

Hi all,

I've tried searching the forums and Google but can't seem to hit the right keywords to get what I'm looking for...

While I understand that UK signalling does not technically impose a speed limit under caution aspects, are there any precedents for it implying a speed limit, or is it down to driver discretion? Else, do any TOCs actually give rules/guides as part of their SOPs with regards to speeds under certain aspects?

Thanks!

 

[Snow1964]

There is no official speed limit (apart from zero at next signal if red).

There are cases of experienced drivers maintaining line speed with double yellows changing to green as they approach, basically they are working out how fast a train about 1 - 1.5 miles ahead is running, so considering the line speed of train ahead

 

[Bigfoot]

Most companies have a driving policy which will make recommendations of speeds to be doing when under cautionary signals/approaching a signal at danger.

Eg. half line speed at the single yellow when in 4 aspect signalling
Or under a certain speed at the aws magnet for a signal at danger.

 

[Inversnecky]

I guess it would be hard to stipulate a precise speed as it depends on the circumstances: obviously if signals are a mile or two apart, you have more leeway in reducing speed than if the signals were a few hundred metres apart near a station.

 

[snowball]

Would it not be reasonable to say that measures like approach control are designed to limit train speed without applying an explicit limit?

 

[43066]

Else, do any TOCs actually give rules/guides as part of their SOPs with regards to speeds under certain aspects?

The only “rule” I’ve been familiar with is max 20mph over the magnet approaching a red. Even this is really more of a ceiling, as there are some sections (eg around the Thameslink core) where even this would be way too high.

Other than that route knowledge comes into it. On our four aspect 125mph sections, as a rule of thumb, I aim for 80mph at YY, 60mph at Y which works fine. On the other hand on the 60mph suburban metro routes I used to sign, 40mph for YY, 30mph Y was a good guide.

The main risk associated with managing speed when approaching signals these days is triggering TPWS (usually set at around 40mph) or TPWS+ (further back, usually set around 65mph), which of course is a safety of the line incident. It is possible to trip grids even when following a braking curve that wouldn’t result in a SPAD, so it pays to drive defensively and not to rush.

 

[Watershed]

Would it not be reasonable to say that measures like approach control are designed to limit train speed without applying an explicit limit?

Correct, as approach control normally uses timers connected to a track circuit (obviously all done digitally in modern installations) to establish that the average speed of the train must be below a certain value, before the signal goes off.

TPWS also results in implied speed limits, as an emergency brake demand will be triggered if trains approach speed limits, buffer stops or red signals too quickly (even if they are theoretically capable of stopping/slowing down in time).

In a few cases - usually where signals are spaced very closely - TPWS+ grids are located on approach to the signal preceding the signal protecting a junction etc., meaning that there is an implied speed limit despite the signal displaying a proceed (albeit cautionary) aspect.

 

[edwin_m]

A route or junction indicator indicates compliance is necessary with the signed permitted speed for that route?

 

[43066]

A route or junction indicator indicates compliance is necessary with the signed permitted speed for that route?

Yep, the driver needs to know what that is, of course (again route knowledge). Flashing aspects denote highest speed diverging route at the forthcoming junction.

 

[karlbbb]

Thanks all - interesting replies! Exactly the kind of thing I was after, such as those very rough, rule-of-thumb type of things. As someone with a passing interest, I do understand it is wildly variable based on routes, signalling areas etc., and mostly down to driver knowledge, but I certainly appreciate the insight!

== Doublepost prevention - post automatically merged: 6 Jan 2022 ==

The only “rule” I’ve been familiar with is max 20mph over the magnet approaching a red. Even this is really more of a ceiling, as there are some sections (eg around the Thameslink core) where even this would be way too high.

Other than that route knowledge comes into it. On our four aspect 125mph sections, as a rule of thumb, I aim for 80mph at YY, 60mph at Y which works fine. On the other hand on the 60mph suburban metro routes I used to sign, 40mph for YY, 30mph Y was a good guide.

The main risk associated with managing speed when approaching signals these days is triggering TPWS (usually set at around 40mph) or TPWS+ (further back, usually set around 65mph), which of course is a safety of the line incident. It is possible to trip grids even when following a braking curve that wouldn’t result in a SPAD, so it pays to drive defensively and not to rush.

If you don't mind my asking, where you say 80 in a YY for example, would you aim to be braked down to 80 as you approach the signal at YY, or would you brake down to 80 or below within that YY section?

 

[43066]

If you don't mind my asking, where you say 80 in a YY for example, would you aim to be braked down to 80 as you approach the signal at YY, or would you brake down to 80 or below within that YY section?

I’d aim to be passing the double yellow at approx. 80mph (brake goes in as soon as I see it) to avoid having to brake excessively harshly. Although “service braking distance” to the red (for the worst performing stock on the route travelling at line speed) starts from the double yellow, there are some places where the red seems to come up rather more quickly than others!

 

[Inversnecky]

I had read that your default brake setting is step 2, but if you do that on seeing a double yellow, is there not a chance you slow down too quickly in too short a distance and have to power up again, then brake again?

Would it be common practice to use step 1 at a double yellow to ensure a gradual decrease of speed to the red?

 

[Llama]

It's better practice to take some positive action at the YY in that circumstance, so that might mean braking as per the driving policy instructions for the train to scrub some speed off and then coast.

You wouldn't brake, then take traction power to increase speed, then brake again.

 

[Inversnecky]

Thank you. So you’d pull off the brakes once you were coasting at the speed appropriate for when you reached the single yellow.

 

[43066]

I had read that your default brake setting is step 2, but if you do that on seeing a double yellow, is there not a chance you slow down too quickly in too short a distance and have to power up again, then brake again?

Would it be common practice to use step 1 at a double yellow to ensure a gradual decrease of speed to the red?

Not all stock has brake steps but, for that which does, step two is indeed often the recommended initial application. Generally it’s best practice to kill the speed early and then back off, especially when braking down from higher speeds.

Thank you. So you’d pull off the brakes once you were coasting at the speed appropriate for when you reached the single yellow.

It’s tempting sometimes to slow right down and crawl towards the red from a fair way off. Especially if (say) you know you’re following a stopper and the signal is likely to clear soon as it leaves the station ahead of you. This isn’t always a good idea as attention can wander and it has been known for drivers to SPAD at low speed. It’s no less serious if it happens at 5mph than at any other speed!

 

[centro-323]

The most fun is coming round a corner at 125mph in a 3-aspect area and finding a single yellow staring at you. This is where it's important not to panic by throwing the brake in too fast, something which will be extremely tempting early in one's training!

 

[karlbbb]

The most fun is coming round a corner at 125mph in a 3-aspect area and finding a single yellow staring at you. This is where it's important not to panic by throwing the brake in too fast, something which will be extremely tempting early in one's training!

Wouldn't have thought the word "fun" would be used there I'd imagine you still need a fair whack of brakes to get to a sensible speed for the (potentially) upcoming danger?

 

[jfollows]

Wouldn't have thought the word "fun" would be used there I'd imagine you still need a fair whack of brakes to get to a sensible speed for the (potentially) upcoming danger?

No, the signal spacing will allow for reaction time and will add a margin on top and still allow the train to come to a stand easily using normal service braking before the next signal, extra braking force shouldn't be required.

centro-323 said this effectively above - over-reaction is wrong.​

== Doublepost prevention - post automatically merged: 7 Jan 2022 ==

However, I'd expect that there would be many places where a single yellow would not be unusual, such as approaching a conflicting junction, and it'd be a little more thought-provoking if the single yellow were not in such a location, where the signal would more normally be expected to be green.

 

[M&NEJ]

From memory I thought there had been an example of a speed signalling system in the UK. I recalled L&Y as a possible suspect and tracked this down about the Midland Railway introducing so-called speed signalling on the former L&Y main line around Mirfield. Happy reading (I would hate to have to cope with this if I were a driver)...!

LMS Speed Signalling – The Signal Box

signalbox.org

 

[Annetts key]

Under the current system, no speeds can be set, as the braking performance differs considerably between different train types.

When the positioning of the signals in track circuit block areas is being done, careful consideration of numerous variables are taken into account, including but not limited to:

• Maximum/permitted line speed,
• Gradient of the line,
• Breaking performance of the worse case type of train type that is permitted to use the line,
• The maximum number of trains that are planned to use the line,
• The position of signals in relation to junctions, so that there is sufficient distance for the overlap before the points,
• Positions of signals to stations if all passenger trains are planned to stop at these stations.

Permanent and junction speed restrictions may also be taken into account.

The overall result is that drivers are always given sufficient warning (yellow or double yellow aspect) so that they have sufficient service braking distance before arriving at a signal displaying a red aspect.

For trains (modern passenger) that have much better brakes than older train designs, a differential (maximum/permitted) line speed may be used. That is, the trains with better braking performance have a higher line speed compared to other trains.

Approach release (approach control) for junction signals is done on a combination of either the track circuit (or axle counter or other train detection device) detecting the train, or that plus a timer. It’s a compromise, as the signalling has no way to know what type of train is present, so it has to work for all trains.

TPWS and TPWS+ installations are also a compromise for the same reasons. They have to be installed to take account of trains with traditional braking performance, hence drivers of modern trains with better brakes have to drive defensively otherwise they risk tripping it and coming to an embarrassing (to some) halt in the middle of nowhere.