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Non stopping stations on LU

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Antman

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I've often wondered why trains have to come almost to a stop rather than just passing through at line speed as would happen on the 'big railway', something to do with the tripcock?

On Saturday I was on the eastbound platform at Sloane Square and the westbound platform was closed because of building work upstairs. Most trains were approaching at a crawl, far slower than if they were stopping at the station normally, and some were sounding the whistle even though there was nobody on the platform. One particular train came in a bit faster and came to an abrupt halt with just the front two coaches alongside the platform, it sat there for a minute or two and I noticed the driver on the phone. Had the train been 'tripped' and he had to get permission to continue?
 
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Peter Mugridge

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It's something to do with the signal spacing and overlap being set for trains calling at the station.

On the ATO lines, particularly the Northern, I have several times passed through closed stations at almost the normal line speed. These lines of course use a completely different signalling system.
 

Mojo

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I've often wondered why trains have to come almost to a stop rather than just passing through at line speed as would happen on the 'big railway', something to do with the tripcock?
The conventional signalling system on the Underground is designed so that if a train (in proper mechanical order and travelling at or under the speed limit) were to pass a signal at danger it would be tripped and come to a stand within the overlap so that it would not be at risk of collision or derailment. This is unlike on the mainline where overlaps are a fixed length (I believe the nominal length is 180 metres but this can be more or less depending on local setup and signal type) and if a train passes a signal at danger it may or may not stop within the overlap.

Overlaps at most station starter signals are therefore set to assume that a train is going past at 5mph, meaning the overlap is much shorter than for an ordinary running signal in section. At some locations the overlaps are longer to enable trains to pass stations at higher speed, for instance Turnham Green on the Fast lines trains may pass at 45mph, Chiswick Park - Ravenscourt Park on the Local lines trains may pass at 20mph, and so on.
 
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pdeaves

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As a general point, capacity reduces when you try to mix 'fast' and 'slow' trains. By (almost) stopping, the speed profile is much closer to other trains, reducing the chance of catching up the one in front and keeping capacity at its best.
 

Mojo

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On Saturday I was on the eastbound platform at Sloane Square and the westbound platform was closed because of building work upstairs. Most trains were approaching at a crawl, far slower than if they were stopping at the station normally, and some were sounding the whistle even though there was nobody on the platform. One particular train came in a bit faster and came to an abrupt halt with just the front two coaches alongside the platform, it sat there for a minute or two and I noticed the driver on the phone. Had the train been 'tripped' and he had to get permission to continue?
Was this just before 10pm?

A train won't get tripped if it is going too fast for a closed platform in these circumstances.
 

bramling

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I've often wondered why trains have to come almost to a stop rather than just passing through at line speed as would happen on the 'big railway', something to do with the tripcock?

On Saturday I was on the eastbound platform at Sloane Square and the westbound platform was closed because of building work upstairs. Most trains were approaching at a crawl, far slower than if they were stopping at the station normally, and some were sounding the whistle even though there was nobody on the platform. One particular train came in a bit faster and came to an abrupt halt with just the front two coaches alongside the platform, it sat there for a minute or two and I noticed the driver on the phone. Had the train been 'tripped' and he had to get permission to continue?

The main reason is simply that signal overlaps are generally calculated on the basis that every train starts from a platform from a rest. If a train were to pass through a platform at full speed then the calculated overlap lengths no longer hold true. It's also undesirable as many station starter overlaps are of nominal length only - on the assumption that a misjudgement is more likely than a driver making no attempt to stop at a platform at all. There is a caveat however that this will not apply if there is a signal berth in advance of the platform where a train can be brought to a stand. Certain designated platforms are designed for running through at higher speed, some might be 25 mph for example, others might be full speed.

Another reason for running slowly through platforms is that the starting signal may not have sighting from full speed, as starting signals generally aren't provided with a repeater. A platform repeater, if available, is not sighted for being seen by the train operator, plus unlike a normal repeater signal they will show green even if the associated stop signal still displays a dual aspect. Repeater signals are provided for starting signals at the specific platforms where trains are authorized to pass through at higher speeds, an example of where this may be found is between Ealing Common and South Harrow.

The difference between LU and mainline is that LU's overlaps are designed to be able to allow for a train to stop, whereas on the mainline they are normally provided solely to cater for a misjudgment. The lower speeds on LU make this arrangement feasible, whereas on the mainline it wouldn't be feasible to provide 125 mph overlaps whilst maintaining line capacity.

The ATO lines are different as with continuous ATP there's no need for any restriction, however evidently LU feel that there should still be some kind of restriction - for safety of people on platforms plus other issues like displacement of tunnel dust. On the Central Line there's nothing to physically stop a train going through a platform at full speed, however the rules say otherwise.

Historic practice was that drivers were generally instructed to stop short of a platform if the starting signal was at danger, however over time many starting signals have gained timing sections, so this practice has tended to fall into disuse. It doesn't apply on the TBTC lines due to the design of the system.
 

Antman

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Was this just before 10pm?

A train won't get tripped if it is going too fast for a closed platform in these circumstances.

No it would have been about 8pm.

Of course there might be some other reason why the train stopped, passenger alarm maybe, that was just my guess as to what had happened.

Many thanks to everyone who has replied.
 

rebmcr

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Historic practice was that drivers were generally instructed to stop short of a platform if the starting signal was at danger, however over time many starting signals have gained timing sections, so this practice has tended to fall into disuse.

Is that analogous to Approach Control?
 

Dstock7080

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the westbound station starter at Sloane Square is automatic FDX768, which hasn’t yet been fitted with a timer, so will clear after the passage of a train.
Early on Saturday morning some trains stopped at the platform as no notices were displayed and the new S Stock CSDE equipment cannot be switched off.
 

bramling

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Is that analogous to Approach Control?

Yes. Typically there might be a timer of 15 seconds on the platform track circuit.

This proves that a train starting away from the station is starting from or near to having been at a stand.

It's not something that's been applied retrospectively, and is something that only started being done as routine from the 1980s onwards. So every starting signal on the Bakerloo Line has a timing section, but on the Piccadilly Line whose signalling is generally a decade or so earlier most don't. Some places have gained them even on old schemes, for example where a station has been closed for a period of time (Mornington Crescent was an example of this), or where there's been some local changes to the signalling and therefore things have had to be made compliant with current standards (Euston Square is a recent example of this).
 

philthetube

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A drawback with this is that it increases the Spad risk, if it is unusual for a signal to be red on arriving at a station drivers notice it, if it is always red there is a risk that they may not notice on the occasion when if fails to clear.
 

jumble

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The conventional signalling system on the Underground is designed so that if a train (in proper mechanical order and travelling at or under the speed limit) were to pass a signal at danger it would be tripped and come to a stand within the overlap so that it would not be at risk of collision or derailment. This is unlike on the mainline where overlaps are a fixed length (I believe the nominal length is 180 metres but this can be more or less depending on local setup and signal type) and if a train passes a signal at danger it may or may not stop within the overlap.

Overlaps at most station starter signals are therefore set to assume that a train is going past at 5mph, meaning the overlap is much shorter than for an ordinary running signal in section. At some locations the overlaps are longer to enable trains to pass stations at higher speed, for instance Turnham Green on the Fast lines trains may pass at 45mph, Chiswick Park - Ravenscourt Park on the Local lines trains may pass at 20mph, and so on.

Interestingly when we travelled "Steam back on the Met" 2013 we came non stop down the Local lines between Moor Park and Harrow
I GPS timed us at 50 odd MPH and do not recall any slow down at the stations in between.

Is this because the Met always considered itself a real railway and therefore followed big raiilway signalling practice ?

Regards Jumble
 

Mojo

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Interestingly when we travelled "Steam back on the Met" 2013 we came non stop down the Local lines between Moor Park and Harrow
I GPS timed us at 50 odd MPH and do not recall any slow down at the stations in between.

Is this because the Met always considered itself a real railway and therefore followed big raiilway signalling practice ?

Regards Jumble
Having had a look at the signal scale plans; not as far as I can see; signals still have variable overlaps and thus follows conventional signalling practice. For instance the overlap for the station starter at Northwood Hills is 466.1metres[1]

As per the Metropolitan line supplement Northwood - North Harrow inclusive stations may be passed at 50mph. Moor Park local line platforms may be passed at 15mph.

[1] Having said that, certain parts of the Met line have signals that are rarely seen on the rest of the Underground but which would be familiar to staff familiar with mainline signalling experience; such as 4 aspect signals (green, double yellow, yellow, red), banner repeaters, and OFF indicators.
 

jumble

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Having had a look at the signal scale plans; not as far as I can see; signals still have variable overlaps.

As per the Metropolitan line supplement Northwood - North Harrow inclusive stations may be passed at 50mph. Moor Park local line platforms may be passed at 15mph.

Very informative and thanks for confirming I am not senile
Many thanks Mojo..

Jumble
 

bramling

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Having had a look at the signal scale plans; not as far as I can see; signals still have variable overlaps and thus follows conventional signalling practice. For instance the overlap for the station starter at Northwood Hills is 466.1metres[1]

As per the Metropolitan line supplement Northwood - North Harrow inclusive stations may be passed at 50mph. Moor Park local line platforms may be passed at 15mph.

[1] Having said that, certain parts of the Met line have signals that are rarely seen on the rest of the Underground but which would be familiar to staff familiar with mainline signalling experience; such as 4 aspect signals (green, double yellow, yellow, red), banner repeaters, and OFF indicators.

The local lines are simply designed on the basis of being able to proceed at full speed, hence longer overlaps. The usual trade-off between speed and capacity doesn't apply on a section of line which carries a comparatively infrequent service.

Without checking, I'd suggest the 15 through Moor Park is more likely to be to do with that being the location of tripcock testers.
 

Mojo

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Without checking, I'd suggest the 15 through Moor Park is more likely to be to do with that being the location of tripcock testers.
It's only the Northbound Local that has a tripcock tester from memory. The overlaps are also shorter here, necessitating the reduced speed (not that adding much will aid things that much due to the PSRs in the area as a result of the junctions/curve).
 

Mutant Lemming

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You could actually run faster on the local than the fast in the days of 70 mph running but needed to be extra careful approaching Harrow S/B as there was a lack of repeaters for the junction stick.
 
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