Car struck by a train at Yarnton (nr Tackley) on 02/01/2013 & AHB Crossing Discussion

[SussexMan]

I think some clarification is needed.

AHB level crossings are operated by track circuits, supplemented by treadles. Since the timings at the crossing are critical, the treadle ensures that the strike-in track circuit becomes 'occupied' immediately the first axle of a train enters it, irrespective of any factors (e.g. railhead contamination) that might cause a slight delay in the track circuit being shunted.

A treadle is just a rail-mounted contact (or electronic equivalent) that can detect that the front of a train has reached a particular point. It is not an axle counter and it doesn't measure anything.

I can't see that it would be possible to operate an AHB crossing by treadles alone. A treadle cannot detect that the rear of the train has cleared the crossing.

If axle counters are used instead of track circuits, treadles are not necessary.

Thank you for that informative post. Are there 2 strike in points? I assume an Advance one is needed which will keep the barriers down if they are already down. There must be a set distance between the two depending on line speed to ensure that the barriers can rise and a certain time elapse before the yellow lights start again. Single lines excepted!

 

[Railsigns]

Are there 2 strike in points? I assume an Advance one is needed which will keep the barriers down if they are already down. There must be a set distance between the two depending on line speed to ensure that the barriers can rise and a certain time elapse before the yellow lights start again. Single lines excepted!

Originally, treadles were installed at two positions on the approach to an AHB crossing (apart from on single lines, as you say), one at the 'strike-in' position, and another further away to keep the barriers down in the 'another train coming' situation. Modern practice is to only provide the outer treadles, and delay the operation of the crossing for a few seconds until the train reaches the normal strike-in position.

 

[TDK]

I think some clarification is needed.

AHB level crossings are operated by track circuits, supplemented by treadles. Since the timings at the crossing are critical, the treadle ensures that the strike-in track circuit becomes 'occupied' immediately the first axle of a train enters it, irrespective of any factors (e.g. railhead contamination) that might cause a slight delay in the track circuit being shunted.

A treadle is just a rail-mounted contact (or electronic equivalent) that can detect that the front of a train has reached a particular point. It is not an axle counter and it doesn't measure anything.

I can't see that it would be possible to operate an AHB crossing by treadles alone. A treadle cannot detect that the rear of the train has cleared the crossing.

If axle counters are used instead of track circuits, treadles are not necessary.

From a drivers point of view when working over certain lines there are only certain AHB level crossing that need to be passed at caution and are locally monitored by a hand signaller and this is when there is a TC failure, if they are all track circuited then why only certain crossings need to be transitioned at caution and not all? The line I am talking about is the single line from Wrexham to Saltney Junction and I knpw for certain that at least 2 of these AHB crossings are treadle operated after a train passes for the barriers to be raised?

 

[Railsigns]

From a drivers point of view when working over certain lines there are only certain AHB level crossing that need to be passed at caution and are locally monitored by a hand signaller and this is when there is a TC failure, if they are all track circuited then why only certain crossings need to be transitioned at caution and not all? The line I am talking about is the single line from Wrexham to Saltney Junction and I knpw for certain that at least 2 of these AHB crossings are treadle operated after a train passes for the barriers to be raised?

As Tomnick explained in post #121, a single track circuit failure is only likely to affect one AHB LC in the section. All the other crossings will operate normally.

 

[TDK]

As Tomnick explained in post #121, a single track circuit failure is only likely to affect one AHB LC in the section. All the other crossings will operate normally.

In the situation with the single line I am refering to the pway have told me that there are track circuits eiether end at CN section signal and at Chester#s first signal and one other at Balderton AHB, if the one at Balderton fails the signals can still be cleared but the crossing must be monitored, if one of the track circuits fail either end then all the track circuits on that line will all be affected and pilot working will be implimented. Can you send me a source I can read that clearly states that all AHB level crossings are track circuited?

 

[Railsigns]

In the situation with the single line I am refering [referring] to the pway have told me that there are track circuits eiether end at CN section signal and at Chester#s first signal and one other at Balderton AHB, if the one at Balderton fails the signals can still be cleared but the crossing must be monitored, if one of the track circuits fail either end then all the track circuits on that line will all be affected and pilot working will be implimented [implemented].

This sounds like a case of one long axle counter section provided for signalling purposes on the single line (apart from the track circuits at the extremities), with some local track circuits overlaid within the axle counter area purely for level crossing control.

Can you send me a source I can read that clearly states that all AHB level crossings are track circuited?

Nothing that I could readily direct you to gives such a clear statement as that. It's all in the relevant standards though.

 

[Tomnick]

In the situation with the single line I am refering to the pway have told me that there are track circuits eiether end at CN section signal and at Chester#s first signal and one other at Balderton AHB, if the one at Balderton fails the signals can still be cleared but the crossing must be monitored, if one of the track circuits fail either end then all the track circuits on that line will all be affected and pilot working will be implimented. Can you send me a source I can read that clearly states that all AHB level crossings are track circuited?

Firstly apologies - I misread your earlier post mentioning pilotman working, so thought we were talking about SLW on a double line rather than Working by Pilotman on a single line. My comments regarding the lack of wrong-direction controls are clearly irrelevant!

It seems that there are two separate issues here - the failure of one of the TCs on the single line itself would require Working by Pilotman to be introduced (RB Module P2 refers), obviously through the whole section, but that doesn't mean that other TCs in the section have necessarily failed too - there's no reason why most or all AHBs couldn't continue to operate normally. See Module TS9 for the list of circumstances under which an AHB must be taken on local control - the failure of one of the TCs controlling the crossing sequence would cause the barriers to remain down (and thus lead to a 'failed' indication in the box), meaning that Drivers would have to be cautioned and, eventually, the crossing would probably be taken on local control. If, as Railsigns suggests, the controlling TCs for the AHB don't form part of the system for maintaining the integrity of the single line (and probably aren't indicated in the box), there should be nothing stopping trains otherwise being signalled normally over the section.

 

[phonoplug]

I can't see that it would be possible to operate an AHB crossing by treadles alone. A treadle cannot detect that the rear of the train has cleared the crossing.

If axle counters are used instead of track circuits, treadles are not necessary.

I know nothing about the inner workings of railways, so excuse me if this sounds dumb, but surely a threadle can be connected to a counter circuit to count the number of wheels/axles that pass. The first wheel that passes triggers the crossing lights etc. Past the crossing another threadle counts the wheels leaving the area and decrements the counter, and then the counter goes back to zero the crossing barriers go up.

 

[Tomnick]

Axle counters can be, and are, used to do just that (making treadles unnecessary, as Railsigns explains). I doubt that treadles themselves could be relied upon to count axles though - after being depressed by one wheel, would they return to their normal position quickly enough to catch the next on a fast train?

 

[455driver]

I know nothing about the inner workings of railways, so excuse me if this sounds dumb, but surely a threadle can be connected to a counter circuit to count the number of wheels/axles that pass. The first wheel that passes triggers the crossing lights etc. Past the crossing another threadle counts the wheels leaving the area and decrements the counter, and then the counter goes back to zero the crossing barriers go up.

But if the (out) treadle missed an axle the barriers would stay down!

treadles are not axle counters they are a simple switch activated by the first wheel over them.

 

[TDK]

This sounds like a case of one long axle counter section provided for signalling purposes on the single line (apart from the track circuits at the extremities), with some local track circuits overlaid within the axle counter area purely for level crossing control.

As far as I am aware there are not any axle counters in that section. CN box is a satalite AB box and Chester a PSB between CN section signal and CR ist signal is CN block section as such, possibly there are track circuits there but accordong to the pway there is not except at one AHB.

 

[Tomnick]

There must be one or the other continuously through the section. I'd talk to the S&T rather than the p-way if I were you!

 

[TDK]

There must be one or the other continuously through the section. I'd talk to the S&T rather than the p-way if I were you!

I will have a chat with the S & T and post back on this one.

 

[Railsigns]

Okay, I've checked: There is continuous track circuiting between Chester and Wrexham (no axle counters).