What prevents points changing under trains?

[Fay Singpoint]

Three notable accidents worth reading about where points have moved when they shouldn't have to derail a train / cause a collision are -

 

1973 - Ealing Broadway

1969 - Connington South (near Peterborough)

1927 - Hull Paragon

 

1976 - Hither Green should make a good read for the OP.

[The Stationmaster]

The only axle counters at Euston are in Primrose Hill Tunnels. Everything else is track circuits.

 

Thanks for that - there was me wondering why they were being included at St Pancras when elewhere they were going in for axle counters.

[micknich2003]

The Paragon accident was caused by the signalman disobeying the rules. He put a signal back to danger before the entire train had cleared the Facing Points in Advance of the train. This released the mechanical interlocking and allowed the points to be pulled, which he did in error when in his haste he grabbed the wrong lever. Pure bad luck, he had about 2 secs to do it before the leading wheel of the engine was on the Lock Bar of the Facing Points in question. I have the full report here, about 28 pages from memory.

Of couse if in 1903/05 the North Eastern Rly had invested in a few Track Circuits, the incident would have been imposible. The LNER soon put a few in, Hindsight is a wonderfulll thing.

Attached a few pictures, I have others. Mick.

[The Stationmaster]

The Paragon accident was caused by the signalman disobeying the rules. He put a signal back to danger before the entire train had cleared the Facing Points in Advance of the train. This released the mechanical interlocking and allowed the points to be pulled, which he did in error when in his haste he grabbed the wrong lever. Pure bad luck, he had about 2 secs to do it before the leading wheel of the engine was on the Lock Bar of the Facing Points in question. I have the full report here, about 28 pages from memory.

Of couse if in 1903/05 the North Eastern Rly had invested in a few Track Circuits, the incident would have been imposible. The LNER soon put a few in, Hindsight is a wonderfulll thing.

Attached a few pictures, I have others. Mick.

Which fully illustrates the point I mentioned about the Rules regarding the working of signals being as relevant as the equipment.

[Southernman46]

Thanks - Hither Green is a new one to me - I do remember the switch diamonds at the London end of the station - they were eventually removed and replaced with ordinary crossings but the detection circuitry remained in the interlocking.

 

Ah - different SD's having read it.........

[RJS1977]

And the 1973 Ealing accident was caused by a battery box flap on a Western coming undone and catching the moving part of an electrically-interlocked point motor (the electrical interlocking just stopped the signalman changing the point, not stopping it being changed by other means).

[Southernman46]

Yes - the point being (no pun intended) just to point Again no pun etc.....) the OP in the direction of some accident reports that contain technical info on what stops point moving under trains and how this can be defeated in different ways

[Oldddudders]

1976 - Hither Green should make a good read for the OP.

It quite spoiled my day! Most people had already been there about 12 hours when it happened, as had I. Have you got a link to the report? The story I got at the time was that a mod had been sent out by the drawing office, but not received by the installers. I do remember bringing in the assisting engine - and having to replace the dets after we'd (intentionally) run over them.

[The Stationmaster]

It quite spoiled my day! Most people had already been there about 12 hours when it happened, as had I. Have you got a link to the report? The story I got at the time was that a mod had been sent out by the drawing office, but not received by the installers. I do remember bringing in the assisting engine - and having to replace the dets after we'd (intentionally) run over them.

 

Google is your friend, the pdf lies within this page http://www.railwaysarchive.co.uk/docsummary.php?docID=768

[Eggesford box]

The requirement as Mike and Mick have mentioned as to the working of signals is still in the rule book

 

'4.4.1

You must replace a stop signal to danger:

 

.as soon as the last vehicle of the train has passed the signal,or

 

.where there are points facing to the movement, as soon as the last vehicle is clear of the points.'

 

As more electrical controls where introduced circuits where introduced so that once a signal had been cleared the facing points ahead of it where still locked if a signalman replaced the signal to danger before a train had passed the signal. Once the signal had been replaced to danger then the points would have to time out, typically 2 or 4 minutes, before they could be moved. In some cases the circutry was hidden away but the Western at least seemed to go for timing out relays in big glass jars like old fashioned sweet jars, mounted on the block shelf, where the signalman had to move the pointer around to start the sequence.

 

Some points where also locked by the signal in advance of them as part of the interlocking in the lever frame (backlocking on the western though I daresay it was known by other terms in other areas). This was to stop the signalman being to hasty resetting the road behind a train as it pulled out of a yard or across a crossover for instance and derailing the last vehicles of the train.

[beast66606]

Some points where also locked by the signal in advance of them as part of the interlocking in the lever frame (backlocking on the western though I daresay it was known by other terms in other areas). This was to stop the signalman being to hasty resetting the road behind a train as it pulled out of a yard or across a crossover for instance and derailing the last vehicles of the train.

 

Rear locking on the LNWR

[Edwin_m]

1976 - Hither Green should make a good read for the OP.

 

As we're now onto the related topic of points being incorrectly set in front of approaching trains, here's another much more recent one with considerable similarities.

 

http://www.raib.gov....er_junction.cfm

 

It appears a wiring fault was introduced during some modifications, and not picked up due to inadequate testing. This had the effect that although the interlocking operated both motors of the switch diamond correctly, the detection circuit was falsely made if one of the switches was in the wrong position.

 

Everything seemed OK for several months until the motor was isolated during engineering work and not re-connected afterwards. This left the switch unable to move, but due to the previous fault the signals could be cleared for moves needing it in either setting. This was discovered when a train trailed through and damaged the switches. Had things been slightly different, a train might have been abruptly diverted at 100mph, causing a severe derailment.

[Oldddudders]

The only axle counters at Euston are in Primrose Hill Tunnels. Everything else is track circuits.

The axle counters were introduced about a decade ago in a desperate search for reliability, with the Pendolinos getting closer by the day, and iffy performance by TCs in the tunnels, despite an awful lot of effort by maintenance staff. I assume they have been a success.

 

Anent Hither Green, there is mention of the Inspector - who I think was Sydney T Ball, known as "Shave off!" because that was as close to swearing as he ever got! - using the Independent Point Switches. These were provided on panels of that era, as a means of locking points in position when route-proved interlocking was disabled by a failure, and are worth mentioning in this thread. I am not clear how this is achieved now we have moved away from panels and switches.

 

ISTR in the late '70s, there was an S&T initiative on "Standardisation of Approach Locking", which is also relevant, perhaps. Can anyone throw any light on what this sought to achieve?

[edcayton]

How do the axle counters work? Do they count "x" axles before and then not release the points unti "x" axles have gone past the next counter?

 

Ed

[Siberian Snooper]

I believe that axle counters are used along the sea wall, Dawlish Warren to Tiegnmouth, due to the ravages of the sea at times disrupting track circuits.

 

SS

[Edwin_m]

Independent Point Switches. These were provided on panels of that era, as a means of locking points in position when route-proved interlocking was disabled by a failure, and are worth mentioning in this thread. I am not clear how this is achieved now we have moved away from panels and switches.

 

IECCs have three buttons for normal, centre and reverse, which will lock a point when operated with the screen cursor in the relevant place. As with panels these constrain route setting - unless the point is in "centre" then routes that need to move the point will not be available but it is still possible to set routes which use the point in its current setting. When no route is set they provides a means of moving the point individually, but they will not move a point that is locked by a route or an occupied track. Then it's a case of sending someone out to crank the point by hand (who hopefully won't do so if there is a train standing on it!).

 

These can also have an electronic "reminder device" attached, which does the same thing as the magnetic switch covers used on panels (to remind the signaller there is a good reason for this point being locked and to think twice before overriding/removing it).

 

How do the axle counters work? Do they count "x" axles before and then not release the points unti "x" axles have gone past the next counter?

 

Basically yes. In the best Brian Hanrahan trandition they count all the axles out and count them back.

 

The counter head can distinguish which direction the wheel is passing (it is actually two detectors close together and monitors the order they are triggered) so should still operate even if a train reverses over the counter head. There is a counter head at each boundary between sections where trains need to be detected (equivalent to the joints between track circuits), which can talk to "evaluator units" on both sides. Each evaluator talks to at least one head (in dead end sidings, two on plain line, three or more with pointwork) and keeps a count of the total number of axles within the section, then feeds an occupied or not occupied status into the rest of the signalling, which treats it in the same way as a track circuit.

 

The problems arise if a particular train doesn't reliably work the counter heads (eg wheel skates as mentioned), but also if there is a power failure. While the counters may remember their status, they cannot "know" whether trains have gone in or out of their sections during the outage. A wide-area power failure potentially means that many sections need to be "swept" manually for trains before the counters are reset.

[royaloak]

Edwin M covered it.

[The Stationmaster]

Some points where also locked by the signal in advance of them as part of the interlocking in the lever frame (backlocking on the western though I daresay it was known by other terms in other areas). This was to stop the signalman being to hasty resetting the road behind a train as it pulled out of a yard or across a crossover for instance and derailing the last vehicles of the train.

Stop signals locking trailing points in rear of them was one of the standard principles in mechanical locking although it only becomes critical if the point is within a train length from the signal in advance. Signals did not normally require facing points in rear of them unless there were any special conditions (e.g. - again - train length considerations which in any case would usually be taken care of by the Locking Bar and the correct working of signals .

[RJS1977]

Stop signals locking trailing points in rear of them was one of the standard principles in mechanical locking although it only becomes critical if the point is within a train length from the signal in advance. Signals did not normally require facing points in rear of them unless there were any special conditions (e.g. - again - train length considerations which in any case would usually be taken care of by the Locking Bar and the correct working of signals .

 

I would have thought this would have been a rare occurrence anyway as wouldn't the signal usually be positioned before the trailing point?

[The Stationmaster]

I would have thought this would have been a rare occurrence anyway as wouldn't the signal usually be positioned before the trailing point?

Not at all unusual to have signals in advance of trailing points as well as the one in rear of that point and protecting it, in fact there must have been hundreds, if not thousands, of such signals on the railways in this country at one time.

[kevinlms]

At one time there was a limit on the distance between bogies on longer vehicles to ensure that at least one wheelset was depressing the bar at any one time. At least one of (I think) the LMS dining cars had an odd appearance as a result as the bogies are set too far in aesthetically speaking.

 

The rules appear to have been less strict on engine release crossovers in terminal platforms. I can think of a few branch terminii where the engine release crossover was not equipped with FPLs, and one (Dalmellington) where the two ends of the crossover were hand points not even co-acting with each other !

 

 

AIUI, this was a reason why many of the restrctions applied to certain coaching stock being banned from certain routes.

 

As for engine release crossovers, the rules didn't need to be enforced, because coaches filled with passengers didn't need to pass over them, only light engines while running around.

 

Kevin Martin

[kevinlms]

 

 

Some points where also locked by the signal in advance of them as part of the interlocking in the lever frame (backlocking on the western though I daresay it was known by other terms in other areas). This was to stop the signalman being to hasty resetting the road behind a train as it pulled out of a yard or across a crossover for instance and derailing the last vehicles of the train.

 

How many of us are guilty of that on uninterlocked layouts?

 

Kevin Martin

[Oldddudders]

How many of us are guilty of that on uninterlocked layouts?

 

Kevin Martin

Some years ago I was watching a US HO layout at an exhibition. A train had a vehicle derail, and blow me if the derailed car didn't then snag a Peco point tie-bar on the opposing track, thus changing the points in front of an oncoming train - result chaos!

[The Stationmaster]

As for engine release crossovers, the rules didn't need to be enforced, because coaches filled with passengers didn't need to pass over them, only light engines while running around.

Kevin Martin

Well 'no, maybe, perhaps' might be the best answer to that because in many cases coaches containing passengers did pass over the points because it was common practice at some stations to push the train back once the engine had run round. In fact on my local branch the release crossover in the by line at the junction not only never had an FPL in its entire existence but after dieselisation had a railcar bogie running over it in the facing direction all through the working day - and that went on for 6 years until the release crossover was finally recovered

[Oldddudders]

And then there are things that can still go wrong http://www.railwaysarchive.co.uk/eventimages.php?eventID=8959&imageID=1247

 

While there do not seem to be any documents relating to this photo, I believe the train to be the 0804 Hove - London Bridge. As you can see, the entire train is in the platform safely - but the rear coach has climbed the platform. Given that this was the morning peak & the train would have been full-ish I find it surprising nothing has been found. "Points split" would seem a likely cause - and clearly no amount of S&T kit will eliminate that.

 

I was early turn in Croydon Control, and the London Bridge Traffic Regulator's words to me were "I've got a f*****g fatal accident up 14*!" Irritating not to find out what really happened!

 

*[Platform number may be mis-remembered]