AHB

[jbilton]

Hi
Got stopped at one the other day....total time from ambers to barriers backup 35 seconds.(Single 153)
I thought someone had said these were operated by a treddle at a fixed distance from the crossing to give 45 seconds warning.
So question is...was the train speeding?
Also saw a Class 60 on oil tanks this evening...plain blue livery (couldn't see the number)
Cheers
Jon

[codrivermike]

There are two AHB's between Thorn & Scunthorpe that have frighteningly short lead times. Certainly well less than the time needed to stop the 66's that run through there. Ironically the next crossing at Godnow Bridge (see my fotopic site) is manually controlled by a man in a hut.

[stephenwiseman]

Excuse stupidity, but what is an AHB?

Thanks
Stephen

[BR7MT]

Automatic Half Barrier crossing.

Basically a barrier goes down across half the road on each side and is triggered by a device next to the track a set distance along the track from the crossing.

Dan

[Nemetode]

The treadle is placed at the point in the track where the fastest possible train will give a minimum of 27 seconds warning, made up of 3 seconds amber, 24 seconds flashing reds. Barriers come down after 8 seconds of the flashing red. So your 35 seconds is actually more than the minimum HMRI requirement.

These times were less when AHB crossings were introduced and were increased after the Hixon crash inquiry. 27 seconds is set to avoid motorists thinking the lights have failed and weaving around the barriers.
Of course the times are set for the fastest train so something trundling along a line cleared for 125 mph at say 35mph will take far longer than 27 seconds to reach the crossing

Some books quote 32 seconds as the minimum warning time but the HSE Ufton derailment report confirms 27 seconds as the minimum.

[jbilton]

Hi
Thanks for that great info.....all sounds about right.....just seemed fast.
The crossing is on a long straight lane....and by the time I'd thought about slowing down...it was all over.
Strange thing is the next crossing down is manned....and this often takes between 3 and 5 mins.
Cheers
Jon

[stephenwiseman]

Thanks for that info!

The crossing next to my station is done manually. It is put down about 4 minutes before the southbound train arrives, is down for the minute its in the station, and the 30 seconds approx that it takes to get the train out the station, accross the crossing and "a safe distance away." The barriers stay down and 2 minutes later the northbound train comes in. As the station can only hold 4 coaches, 4 of them are trailing behind the station, over the crossing. Once that moves on, the barriers finally come up. All in all traffic is blocked for around 10 minutes. Its just as well we don't have half barriers, as people would be weaving between them all the time.

[Nemetode]

Some crossings have a "time out" if a train becomes delayed after striking in the track circuit (eg by a station stop) which allows the barriers to raise again. This has caused problems with "near misses" as this HSE report shows http://www.hse.gov.uk/railways/incidents/cookham.htm but without the "time-out" device and additional signalling the alternative would be drivers dodging the barriers when a train driver expects the crossing to be protected - with the time-out system a flashing red signal shows the train driver that the crossing has reopened and in theory the train driver should stop (presumably there must be some sort of reset, or the signalman is contacted to reset the barriers) so he knows traffic will be crossing.

Must admit I'd never come across the "time out" before and there isn't much about it to explain how it works.

[salopiangrowler]

the system resets as soon as the train stops when the train starts rolling again the barriers drop.

[southcoasttrains]

some AHB's usually have two treddles before the crossing.

Beddingham used to have the Lewes bound set so when a train left Glynde the gates would go down on the second treddle due to the speed.

[Elojikal]

Ed,
Does your dad know any of the people who work in the Berwick box? I was down that way taking pictures the other day and I'm sure one of them was peering out of the window at me like I was a terrorist. lol

[southcoasttrains]

what did he look like?

A good friend of mine works in there.

[Elojikal]

Dunno. Couldn't really make him out from behind the window. He was just peering through and probably thinking "who on earth is that fat b*stard over there taking pictures" lol

[Tomnick]

Sorry, I'm in a pedantic mood tonight .

Some crossings have a "time out" if a train becomes delayed after striking in the track circuit (eg by a station stop) which allows the barriers to raise again. This has caused problems with "near misses" as this HSE report shows
...
Must admit I'd never come across the "time out" before and there isn't much about it to explain how it works.

As far as I'm aware, AHB crossings don't have a timeout, but ABCL crossings (very similar, but locally monitored - i.e. crossing proved to be working properly by the 'Driver's White Light') can (and usually do?) time out after a few minutes. The barriers stay lowered and red (road) traffic lights continue to flash for a while after the crossing times out, just in case the train has started to move towards the crossing, and I think there's a lineside plunger for the driver to re-activate the crossing.

the system resets as soon as the train stops when the train starts rolling again the barriers drop

Someone's been playing MSTS, I see . There's no way the signalling system can detect that the train has actually stopped (until subsequent track circuits aren't occupied).

[Nemetode]

Sorry, I'm in a pedantic mood tonight .

Some crossings have a "time out" if a train becomes delayed after striking in the track circuit (eg by a station stop) which allows the barriers to raise again. This has caused problems with "near misses" as this HSE report shows
...
Must admit I'd never come across the "time out" before and there isn't much about it to explain how it works.

As far as I'm aware, AHB crossings don't have a timeout, but ABCL crossings (very similar, but locally monitored - i.e. crossing proved to be working properly by the 'Driver's White Light') can (and usually do?) time out after a few minutes. The barriers stay lowered and red (road) traffic lights continue to flash for a while after the crossing times out, just in case the train has started to move towards the crossing, and I think there's a lineside plunger for the driver to re-activate the crossing.

the system resets as soon as the train stops when the train starts rolling again the barriers drop

Someone's been playing MSTS, I see . There's no way the signalling system can detect that the train has actually stopped (until subsequent track circuits aren't occupied).

Thanks for that clarification. I thought it was a bit odd that there could be a time out on a fully automatic crossing. According to the HSE report into the near miss where I read about it the time out in that case was nearly four minutes and it said it varies according to location. Four minutes is quite a long time to get delayed on the approach to the crossing, so perhaps it's not as alarming as it looks!