AHBs not interlocked with signals?

[eriks]

Mod Note: split from Lidlington level crossing accident thread http://www.railforums.co.uk/showthread.php?t=139794

Correct.

In 2017 are we really saying that a barrier like this isn't interlocked with signals? Thus, if the barrier/lights control system failed that the train would sail through anyway? Surely if that was the case that could open NR up to all kinds of H&S charges.

This is a general question/comment - and isn't in any way suggesting any possible causes for this tragic accident.

 

[najaB]

In 2017 are we really saying that a barrier like this isn't interlocked with signals? Thus, if the barrier/lights control system failed that the train would sail through anyway? Surely if that was the case that could open NR up to all kinds of H&S charges.

That's why there are limits on line speed where they can be used, and only half barriers to ensure that cars are never blocked in on the crossing.

The cost of linking this type of crossing into the signalling system would be phenomenal as there are a *lot* of them around the network.

 

[Llanigraham]

In 2017 are we really saying that a barrier like this isn't interlocked with signals? Thus, if the barrier/lights control system failed that the train would sail through anyway? Surely if that was the case that could open NR up to all kinds of H&S charges.

This is a general question/comment - and isn't in any way suggesting any possible causes for this tragic accident.

Automatic Half Barrier crossings are generally NOT interlocked with signals, and have been that way for years.

 

[GB]

Has there ever been an accident at an AHB due to wrong side failure?

 

[Llanigraham]

I'm not aware of any.

My Box didn't "control" an AHB, but there was one in the Section, "controlled" by the next Box down. If there was a fault at that AHB, such as lights not working, then they got an Alarm, and we would then Stop and Caution all trains until the crossing was under Local Control, normally by one of the MOM's.

 

[matchmaker]

I've seen stuck AHB barriers. Stuck down. I've never heard of them failing raised, but am happy to be corrected.

 

[Llanigraham]

I've seen stuck AHB barriers. Stuck down. I've never heard of them failing raised, but am happy to be corrected.

Agreed.
From my experience, as soon as anything fails they drop and stay down until taken under local control.

 

[Marklund]

I've seen stuck AHB barriers. Stuck down. I've never heard of them failing raised, but am happy to be corrected.

There's been a few over the years of slow to drop for a few reasons that I can remember.

We had a briefing many many years ago of ensuring bungs were removed when changing any of the hydraulics on the Mark 1s, as one apparently was stuck in and caused a blockage, wrong direction trains going faster than the X40 boards and beating the barriers down, and very strong winds holding up booms.
Road lights all still operated.

But these are very rare.

 

[Stampy]

Agreed.
From my experience, as soon as anything fails they drop and stay down until taken under local control.

True, as this happened to me at Bainton AHB crossing between Peterborough and Stamford once when the barriers suddenly "dropped" about 20 yards in front of me with no wig-wags.

A phone call to Uffington Box via the local phone provided the answer - a power failure in the area - and staff were on the way to manually "talk" traffic past the failure.

Can't remember off hand if the staff over-rode the barriers and lifted them, or we were "talked" across the barriers manually.

However, when I came back a few hours later everything was working again.

 

[furnessvale]

Automatic Half Barrier crossings are generally NOT interlocked with signals, and have been that way for years.

Also it has to be remembered that AHBs were made that way to reduce delays to ROAD traffic not rail.

 

[Tomnick]

Also it has to be remembered that AHBs were made that way to reduce delays to ROAD traffic not rail.

Partly, but I suspect also because the risk of misuse otherwise would be unacceptably high.

 

[QueensCurve]

Automatic Half Barrier crossings are generally NOT interlocked with signals, and have been that way for years.

I don't think they ever were.

One of the issues at Hixon was that the low loader crew and escort were surprised how quickly the train arrived.

 

[Llanigraham]

I don't think they ever were.

But from comments elsewhere it seems that there might be a few places, generally near stations, that there might be some minor interlocking, apparently taking into account whether a train was a "stopper" or not.

That is why I used the words I did.

 

[edwin_m]

But from comments elsewhere it seems that there might be a few places, generally near stations, that there might be some minor interlocking, apparently taking into account whether a train was a "stopper" or not.

That is why I used the words I did.

There are two situations.

The first is when the AHB is near a platform end and it is possible for the signaller to select a control for stopping trains that inhibits the barriers lowering but holds the platform starter signal at danger. I don't recall if the barrier sequence is initiated by timer or by the signaller de-selecting the control, but I'm pretty sure it then has to run through the whole sequence before the signal clears, to be sure the train doesn't arrive too early.

There are also places I believe where there is a signal quite close on the approach to the AHB, and if it happens to be red for other reasons the crossing sequence in inhibited. When the signal is otherwise ready to be cleared it remains at red and the crossing sequence starts. Not sure if it has to run right through before the signal clear or just far enough that the fastest possible train starting from the signal can't get to the barriers too soon.

 

[Railsigns]

There are two situations.

The first is when the AHB is near a platform end and it is possible for the signaller to select a control for stopping trains that inhibits the barriers lowering but holds the platform starter signal at danger. I don't recall if the barrier sequence is initiated by timer or by the signaller de-selecting the control, but I'm pretty sure it then has to run through the whole sequence before the signal clears, to be sure the train doesn't arrive too early.

In some instances, the driver of a stopping train has to press a plunger to start the crossing sequence.

 

[najaB]

I can't remember, are driver's flashing red lights provided at new AHBC installations?

 

[Railsigns]

I can't remember, are driver's flashing red lights provided at new AHBC installations?

No, because that would make them locally monitored, i.e. an ABCL.

 

[Wilts Wanderer]

Bradford-on-Avon has exactly this semi-auto arrangement. Southbound stopping trains arrive with the protecting signal red, and the guard presses a platform plunger to activate the crossing sequence (takes about 20 secs.) Non stop services sail through at up to 70mph on clear signals or otherwise, with the crossing operated by a treadle located in the vicinity of the distant signal ( so therefore no interlocking as you'd never get green distant aspect.)

Auto selection is controlled by Westbury panel. Sometimes the signaller forgets to release to auto for flyers (particularly for passenger trains with non-stop orders if running very late) and they end up having to stop and press the plunger regardless.

 

[najaB]

No, because that would make them locally monitored, i.e. an ABCL.

That's kind of what I was wondering - is there any move to convert AHBs to ABCLs? To my mind they are safer since the train driver knows that the barriers are down (more importantly, that they might not be).

 

[Wilts Wanderer]

I guess one factor NR need to consider is the length of time barriers are down. Full interlocking means they're down for longer, particularly if there is a station prior to the crossing. Barriers that are down for longer increase the chance a motorist will get impatient and jump the crossing. Balancing the safety factors is probably more subtle than people think.

 

[Railsigns]

That's kind of what I was wondering - is there any move to convert AHBs to ABCLs?

No; that would have a terrible impact on train performance.

 

[najaB]

No; that would have a terrible impact on train performance.

Right. Had a moment to think about it - I was forgetting that ABCLs are just upgraded AOCLs so have the (25mph?) speed restriction across them.

 

[Railsigns]

Right. Had a moment to think about it - I was forgetting that ABCLs are just upgraded AOCLs so have the (25mph?) speed restriction across them.

55 mph is the maximum speed possible at both AOCLs and ABCLs. The actual speed will often be lower depending on sighting conditions at the crossing.

 

[Llanigraham]

That's kind of what I was wondering - is there any move to convert AHBs to ABCLs? To my mind they are safer since the train driver knows that the barriers are down (more importantly, that they might not be).

Doubtful, as that would need:
the CCTV equipment fitting to the crossing,
the barriers altering to full barriers,
signals fitted prior to the crossings,
CCTV displays at the Control Centre
AND
more signalling staff!!

Much too expensive!!
--- old post above --- --- new post below ---

55 mph is the maximum speed possible at both AOCLs and ABCLs. The actual speed will often be lower depending on sighting conditions at the crossing.

I think you are wrong.
The speed at my ABCL was far higher than that!

 

[Bald Rick]

I think you are wrong.
The speed at my ABCL was far higher than that!

Then it wasn't an ABCL. max 55 as Railsigns says.

 

[MarkyT]

55 mph is the maximum speed possible at both AOCLs and ABCLs. The actual speed will often be lower depending on sighting conditions at the crossing.

The warning sequences for many locally monitored crossings of these types, where they are next to platforms, are initiated by push buttons operated by train crew. In these cases all trains HAVE to stop, so in addition to imposing speed restrictions, they are often suitable only for lines with a standard all stations stopping pattern.

Getting back to AHBs, the intention of any controls applied to signals nearby is to ensure that the road warning time from the first illumination of the yellow light until the arrival of the train never drops below the agreed minimum for the installation, which is usually 27 seconds today. In all cases, whether operated via stopping/non-stopping controls at the signalbox or otherwise, the only 'interlocking' applied to any signal within the strike-in distance is sufficient delayed clearance to ensure this minimum warning time. The main reason stopping/non-stopping controls are provided is because crossing legislation also requires that no more than a tiny proportion of trains arrives with warning time significantly IN EXCESS of the agreed minimum, to discourage misuse. In some cases where platforms are in the strike-in, the signal before the AHB is only there to enforce the delayed approach and has no other block function.

There was a close relative of the AHB known as the AOCR - Automatic Open Crossing Remote. Used on quieter rural roads this essentially had the same controls and lack of interlocking and local monitoring as the AHB, just without the barriers. These are now no longer permitted and I believe all historic installations have now been converted to AHB or one of the locally monitored varieties.

AHBs (and AOCRs historically) have an alarm circuit to the supervising signalbox which continually monitors power supplies and crossing sequence to ensure the lights and barriers operate in the expected order and within expected times and the system all resets correctly after the passage of each train. Crossing power supplies are backed up by batteries with a design standby life of at least 12 hours at normal duty cycle.

 

[Tomnick]

Doubtful, as that would need:
the CCTV equipment fitting to the crossing,
the barriers altering to full barriers,
signals fitted prior to the crossings,
CCTV displays at the Control Centre
AND
more signalling staff!!

Much too expensive!!

CCTV? Full barriers? Signals? Signalling staff? It doesn't sound like any ABCL that I've ever encountered!

Are you confusing them with MCBs (or MCB-CCTV) perhaps?

 

[Railsigns]

I think you are wrong.
The speed at my ABCL was far higher than that!

I think you may be confused about what an ABCL actually is. For one thing, they don't have a CCTV link back to the control centre.

Max speed is 55 mph.
--- old post above --- --- new post below ---

There was a close relative of the AHB known as the AOCR - Automatic Open Crossing Remote. Used on quieter rural roads this essentially had the same controls and lack of interlocking and local monitoring as the AHB, just without the barriers. These are now no longer permitted and I believe all historic installations have now been converted to AHB or one of the locally monitored varieties.

No; there's still one AOCR on Network Rail infrastructure.

 

[dk1]

Don't forget we also now have AOCL+B too.

AHBs have never been linked to signals & fail in the lowered position. Once this occurs the signaller will eventually stop & caution trains. Line speed applies (up to 100mph) except for wrong direction movements.

 

[RichardN]

Milford is another location that must have a signal inhibited AHB. I've driven a car across the level crossing while a 450 was coming into the platform.