AHB Variants

[Wilkinstown]

Curious as to the variants that exist in relation do automatic half barriers.

In particular what variants are in use that are not monitored from a signal box/signalling centre.

Are there variants which are locally monitored (ie white light)

Are there variants that are designated as driver monitored, if so what is the difference between these and local monitoring ?

Are there variants where the white light is placed some distance on the approach to a crossing ?

Do AHBs normally fail with the barriers up or doen in the event of power loss ? Is there different behaviour for different variants ?

[Mike Hodgson]

An AHB not monitored by a box is called an ABCL (Automatic Barrier Crossing, Locally Monitored).
Has drivers red/white light like an AOCL
Looks the same as an AHB to a road user.

AHB fails barriers down, if they are down too long indication sets an alarm.
It has to be like that as there is nothing to stop an approaching train.
Somebody can be sent out to work it locally if necessary
Power failure - use standby battery. If batteries also fail, crossing fails.

ABCL raises barriers again if train does not arrive at crossing in a reasonable time after strike in, ie fails barriers up.
Train driver has instructions on what to do if the barriers don't lower.
Power failure - drivers white light won't clear - barriers can be hand operated

[Wilkinstown]

Thanks, two further questions, in respect of the indication to the driver that a locally monitored half barrier crossing has successfully operated, is that indication always co-sited with the crossing or can the drivers indication be some distance from the crossing ? What exactly are the indications to the driver - I was aware of the flashing white light indication but you also mention a red indication. Is there normally a red light displayed until the crossing has been triggered and has operated ?

Thanks again for the previous reply.

[AN106]

Yes, there is a flashing red light (LED) which operates whenever the (train) Driver's White Light is not flashing.

[Danny252]

Thanks, two further questions, in respect of the indication to the driver that a locally monitored half barrier crossing has successfully operated, is that indication always co-sited with the crossing or can the drivers indication be some distance from the crossing?

I'm not sure how much variation you're expecting. The indicator has to be close enough that the train has struck in and the crossing has had time to operate before the train arrives, but far enough to be at braking distance so that the train can stop before the crossing if there is a failure - there's presumably not too much leeway between satisfying these two conditions.

(Then again, "at braking distance" might not be the best description, as it seems that it can be that the driver must start braking before he passes the indicator - there was a recent RAIB report on an incident where a driver waited until he was fairly close to an indicator before concluding the crossing was has failed, and was unable to stop before the crossing)

[limitofshunt]

You might find module TW8 of the rulebook of use:

http://www.rgsonline.co.uk/Rule_Book/Ru ... ss%206.pdf

[Mike Hodgson]

When Automatic Open Crossings (ie no barriers) were first introduced, the drivers indication was originally just the flashing white light; the flashing red was a later addition which gave the driver a more positive indication of failure to operate. I think it came in at the same time as the first ABCLs.

The operation becomes more complicated if there is a platform close to the crossing. For example there is an unusual ABCL where the preserved Dean Forest Railway crosses the main A48 Lydney by-pass built only a few years ago. The railway has an unmanned halt just a few yards on the town side of this new road, and after a train has stopped there and is ready to continue, the driver has to push a button to initiate the barriers. For trains going the other way, the crossing indicator is mounted on a bracket on a running signal. The signalman clears the signal with the barriers in the raised position but it is still the approach of the train that triggers the crossing and clears the indicator, so the driver needs both the signal and the flashing white light to cross.

Another example is a minor road near the site of March West box, where a train to Peterborough reaches an AHB shortly after leaving the station. The signalman at March East before signalling a train in that direction has to operate a Stopping/Non-stopping switch. The effect of selecting "Stopping" is to move the strike in point from somewhere before the train even reaches the station to just after the platform.

[edwin_m]

The effect of selecting "Stopping" is to move the strike in point from somewhere before the train even reaches the station to just after the platform.

I presume this is where there is a platform starter signal which is held at danger until the crossing has operated (otherwise there is a definite hazard if the switch is operated for a non-stopping train)? In which case there can be no strike-in as such - is the sequence started by returning the switch to "Non-Stopping" or by some other means?

[Pete2320]

The effect of selecting "Stopping" is to move the strike in point from somewhere before the train even reaches the station to just after the platform.

I presume this is where there is a platform starter signal which is held at danger until the crossing has operated (otherwise there is a definite hazard if the switch is operated for a non-stopping train)? In which case there can be no strike-in as such - is the sequence started by returning the switch to "Non-Stopping" or by some other means?

I don't know about this particular location but at older locations the crossing (in stopping mode) was activated by a drivers plunger whereas in more modern installations there is a time delay before the barrier sequence starts. I too would expect to find a "platform starter" to protect the crossing and the whole issue becomes much more complicated if the signal is also a "block" signal. Not all are.

Pete

[StevieG]

In regard to locally-monitored crossings (driver's flashing white light, etc.), the 'braking distance' mentioned upthread is likely to be short (driver's indicator --> crossing distance, 30-40 yards perhaps) because in my experience, the permissible speed over the crossing is fairly low. and so drivers braking for that on the approach to the driver's indicator, as well as preparing for possible stop if observing it flashing red.

[Late Turn]

I've never known the driver's white light to be located anywhere other than very close to a locally-monitored crossing, even on the relatively fast (all 50mph?) AOCLs on the Skegness branch proper. That seems sensible - the driver needs to check that the crossing is physically clear as well as looking for the flashing white light (and the speed restriction will be determined from the sighting distance to allow that). There's also the possibility of a train being detained on the approach to a crossing, but within the controls - I don't know exactly what the rules say about it, but they time out after a short period, so it's important that the driver knows whether the DWL has extinguished in the meantime.

[Wilkinstown]

Thanks for the information to date.

To refine my query somewhat I am looking for any examples of locally monitored half barrier crossings where the white light is located some distance from the crossing, perhaps 260-300 meters away - i.e. "distance monitored".

[AndyB]

Placing the DWL at a distance (specifically, co-siting with the speed board) was unique to Northern Ireland, and was to eliminate the sighting requirements.

[John]

Northern Ireland Railways LXs are unique to my knowledge with advance DWLs and were known as AHBD's and located at Culleybackey South, Culleybackey North and Culleybackey Station.