German Train Crash

[The Stationmaster]

And whats more that shortcut ("oh the signalling has failed but its nothing serious" attitude "so I will override it") is officially provided for in the form of the Ersatz signal. The nearest equivalent scenario I can come up with in a UK environment is a signalman in a mechanical box unable to clear a signal so he simply sticks a green flag by the signal concerned and wanders off with the driver accelerating away to linespeed upon seeing it.

Actually its been a yellow flag since we revised the Rule Book in 1985 (published I think in 1988).  And it only gives a Driver permission to pass that signal at danger, the next stop signal in advance might be at danger so if the Driver 'whizzes off'  (at other than a Section Signal) he'd be better off carriage cleaning rather than driving trains. 

[phil-b259]

Actually its been a yellow flag since we revised the Rule Book in 1985 (published I think in 1988).  And it only gives a Driver permission to pass that signal at danger, the next stop signal in advance might be at danger so if the Driver 'whizzes off'  (at other than a Section Signal) he'd be better off carriage cleaning rather than driving trains. 

 

Indeed (its one of the many small but significant differences between hand signalmen (note I am not talking about guards here) on the Bluebell (where yellow flags can only be used at distant signals) and the national network where green flags can only be used by barrier attendants at level crossings)

 

However, regardless of the colour of the flags (which have as you say varied over time), it is the overall principle that what I was getting at.

[phil-b259]

And the fact that conflicting POSAs can't be given presumably makes them in one sense safer than the signaller talking a driver past a red signal, as - in principle - drivers on conflicting routes could inadvertently both be given permission to proceed.

 

Correct. Of course it is theoretically possible to have a rear end collision if one train is not being driven in acordance with the POSA rules, but you won't get opposing POSA signals being permitted to clear together. 

[Mark Saunders]

Correct. Of course it is theoretically possible to have a rear end collision if one train is not being driven in acordance with the POSA rules, but you won't get opposing POSA signals being permitted to clear together. 

 

I was under the impression that POSA (proceed on sight authority) was only used on plain line unidirectional lines and that junctions would be treat the same as now with the Driver having to receive verbal authorisation to pass the signal at Danger to proceed over the junction?

[phil-b259]

I was under the impression that POSA (proceed on sight authority) was only used on plain line unidirectional lines and that junctions would be treat the same as now with the Driver having to receive verbal authorisation to pass the signal at Danger to proceed over the junction?

 

There is not an issue with using them at junctions because as I mentioned earlier,  the POSA still requires the route to be locked into the signaling system (i.e. no conflicting ones set with ALL points locked and detected).

 

In fact the first place having them installed on a squadron basis is the Thameslink core which is fully bi-directional and features quite a lot of points and junctions in the POSA area.

 

If the interlocking cannot complete the route setting process because it thinks a conflicting route is set or because a set of points has no detection then the POSA will not work and the signaller will have to revert to the normal procedures (including getting the points clipped up etc then to speak to the driver over the in cab radio to give them authorisation to pass the signal at danger, etc)

 

You could in theory use them on single lines on that basis (not that the UK has any plans to do so) because for a POSA to show a proceed a valid route would have to be set in the interlocking onto the single line - which won't happen if a route has already been set in the interlocking for something coming the other way.

 

Basically POSAs are there as a way of speeding up the process of cautioning drivers past signals due to track circuit failures ONLY. With any other failure, and try as they might the signallers will NOT be able to get the POSA to show a proceed unless everything else the signal normally requires is correct.

 

However as with any form of shunt / call on moves (which is basically what a POSA is to all intents and purposes) if one train has stopped in the signal section and the following one proceeding under POSA signals is not going slow enough, a rear end collision is certainly possible.

 

What you will not get is a head on collision or a side swipe with POSA signals - unlike the situation with Ersatz signals......

[Colin_McLeod]

By way of interest even a verbal permission to pass a signal is not foolproof The accident at Hilden on Northern Ireland Railways 1983 involved the holes of the cheese lining up. In this case there was a faulty track circuit in the location a failed train should have been. The driver of the following train was given permission to pass the protecting signal and was also given the wrong information as to the state of the line. He therefore drove at normal speed and collided with the stationary failed train. Sadly that driver, my friend Herbie Dean was killed

The full report is available here if you click on the pdf link. http://www.railwaysarchive.co.uk/docsummary.php?docID=1169

[The Stationmaster]

By way of interest even a verbal permission to pass a signal is not foolproof The accident at Hilden on Northern Ireland Railways 1983 involved the holes of the cheese lining up. In this case there was a faulty track circuit in the location a failed train should have been. The driver of the following train was given permission to pass the protecting signal and was also given the wrong information as to the state of the line. He therefore drove at normal speed and collided with the stationary failed train. Sadly that driver, my friend Herbie Dean was killed

The full report is available here if you click on the pdf link. http://www.railwaysarchive.co.uk/docsummary.php?docID=1169

 

That sort of thing is what gave rise to the move, getting on for 20 years ago, to come up with a fixed signal which has turned out to be the POSA.  There was considerable debate initially as to whether it was right to provide a fixed signal but CTRL helped the debate along and the slow progress with GSM-R meant that it wouldn't be able to offer a viable alternative method (see below) for a considerable time let alone the alterations it would mean to existing rolling stock.  

 

Hence a fixed signal method was chosen and the original idea was very much one of dealing with track circuit failures without all the route proving which has ultimately been included.  It is perhaps worthwhile considering to what extent that need will change as axle counters become more widespread as the replacement for track circuits as they are, thus far, far more reliable in operation.

 

The GSM-R alternative would have been the use of printed forms transmitted to the driving cab via the radio link which is in many respects probably the best answer.  However it would require traction units to be modified so is only really suitable for routes where dedicated rolling stock is operating (when the system is available).

[Edwin_m]

Hence a fixed signal method was chosen and the original idea was very much one of dealing with track circuit failures without all the route proving which has ultimately been included.  It is perhaps worthwhile considering to what extent that need will change as axle counters become more widespread as the replacement for track circuits as they are, thus far, far more reliable in operation.

The failures with axle counters tend to be where the system has "lost count" for some reason and the track has to be known to be clear before they are reset (how easy or otherwise this may be is something you Mike have alluded to previously!).  I wonder if a POSA would also be useable to get round this type of failure or would it still be necessary to be satisfied there is no train there, in which case just reset the axle counter instead. 

[admiles]

Just by way of an update on this, the signaler/train dispatcher involved has apparently been found guilty of manslaughter and sentenced to three and a half years. 

[BR(S)]

Just by way of an update on this, the signaler/train dispatcher involved has apparently been found guilty of manslaughter and sentenced to three and a half years. 

 

http://www.bbc.co.uk/news/world-europe-38206468