What is highest operational speed that Absolute block can cope with?

[Merseyrailfan]

How fast can trains go, whilst under the Absolute block system of signalling?

 

[InkyScrolls]

How fast could trains go, whilst under the Absolute block system of signalling?

There's no theoretical limit - why would there be?

 

[Chris Hamilton]

While I don’t know if there is a theoretical maximum speed, a lot comes down to how far you place your Distant from your Home signal, in order to create a braking distance. In the past, that would have had a limit based on the maximum length of a wire run and the visibility of said signal from the signal box. Nowadays though, Distants can be electrically powered and also detected (usually for reliability), or even be replaced with a 2-aspect colour light distant.

Of course, that only applies in the area around the Signalbox and in the Distant-Home-Starting signal area. Out on the open main line, the signalling would have no effect on the maximum speed.

 

[cool110]

There's no theoretical limit - why would there be?

There is the 125 mph limit for all forms of lineside signals.

In practice the fastest AB lines are 100 mph in Scotland.

 

[edwin_m]

While I don’t know if there is a theoretical maximum speed, a lot comes down to how far you place your Distant from your Home signal, in order to create a braking distance. In the past, that would have had a limit based on the maximum length of a wire run and the visibility of said signal from the signal box. Nowadays though, Distants can be electrically powered and also detected (usually for reliability), or even be replaced with a 2-aspect colour light distant.

Of course, that only applies in the area around the Signalbox and in the Distant-Home-Starting signal area. Out on the open main line, the signalling would have no effect on the maximum speed.

Distants started being replaced by colour lights before WW2 - the LMS in particular seemed very keen on doing this. This was an easy way of increasing the braking from the distant to the first stop signal and therefore the speed a train could safely travel. As well as avoiding a long wire run, compared to a semaphore it was also much more visible. I think this allowed operation to continue in fog without having to post "fogsignalmen" to place detonators when the signal was at caution.

The spacing of the stop signals within a box's station limits has no bearing on the permitted speed in that area. If the distant is clear the train can run through at line speed and the stop signals will only be at Danger in an emergency. If the distant is not clear then the driver needs to slow to a speed where they can stop at each stop signal, and the signaller will only clear a stop signal if the line is now clear through to the next box, or when they are satisfied the train is under control and able to stop at the next stop signal.

 

[MarkyT]

While I don’t know if there is a theoretical maximum speed, a lot comes down to how far you place your Distant from your Home signal, in order to create a braking distance. In the past, that would have had a limit based on the maximum length of a wire run and the visibility of said signal from the signal box. Nowadays though, Distants can be electrically powered and also detected (usually for reliability), or even be replaced with a 2-aspect colour light distant.

Of course, that only applies in the area around the Signalbox and in the Distant-Home-Starting signal area. Out on the open main line, the signalling would have no effect on the maximum speed.

Distants have always been placed at worst-case braking distance, which can vary considerably depending on max speed, train type, and gradients. Some railways used a standard nominal distance originally, presumably with a lot of margin built in for such variability. Electric motor working of a semaphore distant was also an early option for long pulls, often predating colour light versions.

Some AB boxes are completely colour light today, so in those locations poorer long-distance sighting of semaphores wouldn't be a constraint on speed.

 

[Railsigns]

As speed is increased, there must come a point where it stops being practicable for signalmen to observe the tail lamps of passing trains. That can be overcome by providing continuous train detection through the block sections, but having done that you may as well dispense with the block instruments and implement Track Circuit Block, etc.

 

[Haywain]

There is the 125 mph limit for all forms of lineside signals.

But trains on the GB network have run at higher speeds under conventional signalling.

 

[The Planner]

But trains on the GB network have run at higher speeds under conventional signalling.

But that would need the fifth aspect. How does that work on semaphores?

 

[Haywain]

But that would need the fifth aspect. How does that work on semaphores?

I’m referring to class 91 and HST ‘record breaking’ runs which ran without special signals. The point being that the 125mph limit is considered a safe limit and is not an absolute limit.

 

[D6130]

ISTR that between about 1964 and the mid-1970s trains on the East Coast Main Line were running at 100 mph under Absolute Block regulations with semaphore stop signals between the North London suburbs and Doncaster....with just a few pockets of Track Circuit Block and colour light signalling in the Potters Bar and Grantham areas.

 

[The Planner]

I’m referring to class 91 and HST ‘record breaking’ runs which ran without special signals. The point being that the 125mph limit is considered a safe limit and is not an absolute limit.

But that is a special case, you ignore that in the case of designing normal operations.

 

[Haywain]

But that is a special case, you ignore that in the case of designing normal operations.

That’s pretty much what I said, isn’t it? The OP didn’t specify normal operation or special circumstances.

 

[LYradial]

were there not once places where signal boxes were so close together (Adrian Vaughan and Oxford maybe) that the distants were unable to be placed far enough away and you did not give line clear until you had offered it on and had it accepted.

apply those special rules to every box Via a special traffic notice and you can go any speed you are capable of

 

[MarkyT]

As speed is increased, there must come a point where it stops being practicable for signalmen to observe the tail lamps of passing trains. That can be overcome by providing continuous train detection through the block sections, but having done that you may as well dispense with the block instruments and implement Track Circuit Block, etc.

The cost and maintenance liability to fully 'track circuit' a long block has fallen dramatically with axle counters. Traditional track circuits are typically limited to around a mile in length, so multiple sections were required through a long block with many intermediate electrical cabinets all requiring power and signal cabling, relay circuitry, periodic routine testing and maintenance, faulting, etc. The sections are logically summated so any one occupied or failed shows the overall block occupied. By contrast, axle counters need sensors only at the logical block extremities and nothing between except a communications medium. Even in the late 80s and 90s when axle counters were significantly more costly than individual track circuits, these long block applications made sense as they saved so much intermediate equipment. On more intensively used railways with continuous 3 and 4 aspect signalling, the economics are less clear cut because cabinets and all their support infrastructure are required every mile or so anyway for the signals. This also demonstrates well why single track km alone is a useless measure for estimating signalling complexity and cost.

== Doublepost prevention - post automatically merged: 27 Oct 2024 ==

were there not once places where signal boxes were so close together (Adrian Vaughan and Oxford maybe) that the distants were unable to be placed far enough away and you did not give line clear until you had offered it on and had it accepted.

apply those special rules to every box Via a special traffic notice and you can go any speed you are capable of

When special runs take place at unusually high speed to test vehicles or for publicity, in addition to the special operations measures, the route infrastructure is looked at carefully in advance for any condition issues and to determine what a safe maximum for the run is. Some extra fettling may take place, some renewals brought forward if there's enough notice.

Boxes in close proximity at either end of a station might face the issue you describe for a fast through movement and also the standard block overlap would likely lock up the entire throat at the accepting end. I expect warning acceptance was used in many scenarios at these locations.

 

[Taunton]

were there not once places where signal boxes were so close together (Adrian Vaughan and Oxford maybe) that the distants were unable to be placed far enough away and you did not give line clear until you had offered it on and had it accepted.

apply those special rules to every box Via a special traffic notice and you can go any speed you are capable of

This was known as "slotting", achieved mechanically by combining the operating mechanism of the distant so two (or more) signalboxes had to pull levers for it to come off. There were various ways to do it.

Taunton was exactly like this. You could pretty much see from one box to the next, but speeds were 80mph, and also all main signals featured both Stop and Distant, so there were a fair number to be pulled off for a non-stop through the station, this all dating from the 1930 station remodelling. The distants were electrically worked, and when restored after the train passed the Stop signal would snap back with the characteristic GWR "clang", but the distant would take several seconds afterwards to make it's slow, juddering move back to the horizontal.

Mentioned above are some special runs, which on the WR, and likely elsewhere, were "double blocked", there were particular procedures to keep two full signalbox sections clear ahead before the distant was lowered rather than one. The Royal Train got this, and so I believe did the late 1960s attempts to run accelerated expresses with double-headed Class 37, which in places were running faster than anything before. It was also the plan for the infamous 1964 City of Truro commemoration run with Castles from Paddington to Plymouth in 3 hours 30 minutes, for which all the Taunton boxes were primed for it to flash through faster than Superman - though alas, as it turned out, not on the day ...

 

[InkyScrolls]

There is the 125 mph limit for all forms of lineside signals.

In practice the fastest AB lines are 100 mph in Scotland.

Indeed, but the question was 'What is highest operational speed that Absolute block can cope with?'

 

[LYradial]

This was known as "slotting", achieved mechanically by combining the operating mechanism of the distant so two (or more) signalboxes had to pull levers for it to come off. There were various ways to do it.

Taunton was exactly like this. You could pretty much see from one box to the next, but speeds were 80mph, and also all main signals featured both Stop and Distant, so there were a fair number to be pulled off for a non-stop through the station, this all dating from the 1930 station remodelling. The distants were electrically worked, and when restored after the train passed the Stop signal would snap back with the characteristic GWR "clang", but the distant would take several seconds afterwards to make it's slow, juddering move back to the horizontal.

Mentioned above are some special runs, which on the WR, and likely elsewhere, were "double blocked", there were particular procedures to keep two full signalbox sections clear ahead before the distant was lowered rather than one. The Royal Train got this, and so I believe did the late 1960s attempts to run accelerated expresses with double-headed Class 37, which in places were running faster than anything before. It was also the plan for the infamous 1964 City of Truro commemoration run with Castles from Paddington to Plymouth in 3 hours 30 minutes, for which all the Taunton boxes were primed for it to flash through faster than Superman - though alas, as it turned out, not on the day ...

thanks for that, when my aged brain works again I shall go and look up “slotting”

 

[edwin_m]

were there not once places where signal boxes were so close together (Adrian Vaughan and Oxford maybe) that the distants were unable to be placed far enough away and you did not give line clear until you had offered it on and had it accepted.

apply those special rules to every box Via a special traffic notice and you can go any speed you are capable of

There were various types of double blocking or indicator working which were enforced only by procedure with no protection in the interlocking. One accident caused at least in part by this form of working was the collision of a Pullman at Knowle & Dorridge https://www.railwaysarchive.co.uk/docsummary.php?docID=513

The Pullman express, which comprised nine coaches drawn by a diesel-hydraulic locomotive, was travelling at about 80 m.p.h. as it approached Bentley Heath Crossing box. The driver evidently saw the Knowle and Dorridge Outer Distant signal, which is on the same post as the Bentley Heath Crossing Home signal, at Caution, and he certainly made an emergency application of the brakes. The Outer Distant is, however, only 902 yards from the Home signal, which is not an adequate braking distance from such a speed, and the train passed the latter signal at Danger and collided at about 20 m.p.h. with the leading vehicle of the shunting movement at a point about 377 yards beyond that signal. The special Regulation for the operation of trains where the braking distance beyond a Distant signal is inadequate, which is applicable to the two boxes concerned, had been disobeyed on this occasion by the signalmen.

 

[John Webb]

Distants started being replaced by colour lights before WW2 - the LMS in particular seemed very keen on doing this. This was an easy way of increasing the braking from the distant to the first stop signal and therefore the speed a train could safely travel. As well as avoiding a long wire run, compared to a semaphore it was also much more visible. I think this allowed operation to continue in fog without having to post "fogsignalmen" to place detonators when the signal was at caution.

Indeed. Costly to the railway company in overtime for permanent way workers who usually formed such supplementary signallers. And as it was usually in colder weather when people were burning more coal to cause more fog/smog a rather unpleasant job to do. Besides detonators they also had a hand-lamp to reinforce what the signal aspect was.
Conversion to colour light distants not only got rid of the need for fogsignalmen it also eliminated the maintenance of up to a mile or more of wire and all its supporting pulleys to mechanically operate the signal. It also made life much easier for the signaller although I suspect that was the last thing on the mind of the LMS when they instituted their replacement scheme. Interestingly these 'remote distants' as the LMS called them were given black and white bands on the posts to remind drivers they were distants - important if for some reason the signal had failed.

 

[Merle Haggard]

Indeed. Costly to the railway company in overtime for permanent way workers who usually formed such supplementary signallers. And as it was usually in colder weather when people were burning more coal to cause more fog/smog a rather unpleasant job to do. Besides detonators they also had a hand-lamp to reinforce what the signal aspect was.
Conversion to colour light distants not only got rid of the need for fogsignalmen it also eliminated the maintenance of up to a mile or more of wire and all its supporting pulleys to mechanically operate the signal. It also made life much easier for the signaller although I suspect that was the last thing on the mind of the LMS when they instituted their replacement scheme. Interestingly these 'remote distants' as the LMS called them were given black and white bands on the posts to remind drivers they were distants - important if for some reason the signal had failed.

I recall that some semaphore signals in open country (on ex L.M.S. lines) had these as well - likely, with that consideration, to be distant, but I can't remember whether that was always the case. I think I've read somewhere that it was a policy of an early (1920s) L.M.S. Chief Signal Engineer so the ones I remember lasted well. I think that the logic was to make them stand out when there was no obvious expectation of a signal (apart from route knowledge).

 

[John Webb]

I recall that some semaphore signals in open country (on ex L.M.S. lines) had these as well - likely, with that consideration, to be distant, but I can't remember whether that was always the case. I think I've read somewhere that it was a policy of an early (1920s) L.M.S. Chief Signal Engineer so the ones I remember lasted well. I think that the logic was to make them stand out when there was no obvious expectation of a signal (apart from route knowledge).

A F Bound was appointed in April 1929 as the LMS Signal and Telegraph Engineer. It was he who introduced the policy of converting the distants to colour lights and is also noted for the Mirfield 'Speed signalling' experiment and other unusual signalling on the Euston-Watford 'DC Lines'.
And 'Yes' - mechanical distant posts were also banded, according to "A Pictorial Record of LMS Signals" but the practice was dropped on cost grounds after A F Bound retired.
But I think we may now be deviating a bit from the OP??

 

[Rescars]

.

Mentioned above are some special runs, which on the WR, and likely elsewhere, were "double blocked", there were particular procedures to keep two full signalbox sections clear ahead before the distant was lowered rather than one. The Royal Train got this, and so I believe did the late 1960s attempts to run accelerated expresses with double-headed Class 37, which in places were running faster than anything before. It was also the plan for the infamous 1964 City of Truro commemoration run with Castles from Paddington to Plymouth in 3 hours 30 minutes, for which all the Taunton boxes were primed for it to flash through faster than Superman - though alas, as it turned out, not on the day ...

Does anyone know, would double-blocking have been any more or less complicated on a line operated by Sykes Lock and Block?

Reverting to the OP, there must be a top speed if the AB involves a single line requiring token exchange. At the point of exchange this is surely a pretty low speed if the exchange is made manually, though it could be cosiderably higher if Manson's or similar apparatus were used.

 

[Railsigns]

Reverting to the OP, there must be a top speed if the AB involves a single line requiring token exchange.

Although AB (with interlocked block instruments) has occasionally been used to control single lines, AB never involves token exchange. If tokens are involved, then a completely different set of regulations must apply, e.g. electric token block.

 

[edwin_m]

Does anyone know, would double-blocking have been any more or less complicated on a line operated by Sykes Lock and Block?

I don't think it depends on what block system is in use, as long as the signaller can refuse the train if the previous train has not yet passed the next box.

 

[Harpo]

The pure answer is that AB itself imposes absolutely no speed limit. The limitations come from…

- How the signalling is configured,
- What speed the infrastructure (including crossings of all sorts) can support, and,
- What the human beings in the process can safely and reliably do.