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Independent Position Light Signal Replacement Joint
#1
Hi,

I have a query related to placement of replacement joint for Ground Position light(GPL) in the scheme plan(Not for Module 2, Signalling the layout).

If it is placed less than 5.5m from the GPL, is it required to have last wheel replacement facility or some form of facility to delay the replacement of signal.

I have seen a case where the replacement joint is placed less than 5meters from the signal(GPL), but there was no delayed replacement for that signal.

Can someone clear this query.

Many Thanks in advance.

P.S. Network rail standards says that the replacement joint should be around 5.5m to 20m from the signal(Main) but not clearly mentioned about the GPL.
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#2
I am aware that for a signal with a TPWS TSS (or ATP I believe) there can be "self reversion" problems if the joint is closer than 5.5m from the signal. If the joint has to be closer than this we put Delayed replacement - "last wheel or second track" (which is $40 on the first track on control tables).

Self reversion is when the train occupies the 1st track circuit, replaces the signal to red and THEN travels over the (now energised) TPWS, which therefore acts as if it were a SPAD and stops the train.

If your GPL has TPWS then delayed replacement would be applied. If it does not, ... Not sure. I can think of cases where delayed replacement has been applied anyway but I think it would not be a requirement.
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#3
Hi,

Thanks alot for the reply.

GPL in the scenario, which i referred does not have TPWS.

If the replacement is placed less than 5m from the GPL, a driver traversing the route observes a Red aspect on the signal as soon the train occupies the first track(replacement track), which should not happen. The related control table for that route does not have dealyed replacement facility($40)

Please correct me, if am wrong.

Many Thanks in advance.
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#4
Just clarifed this with a colleage. There are some types of rolling stock where the driver's seat is far enough behind the first axle that (s)he could see the signal replaced to red. So it's not just for TPWS cases.

If the Control Table you're looking at is "real world" there are a number of reasons why there might not be $40 on the first track, such as it's not been designed to modern standards.
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#5
Thanks again for ur reply.

You are correct that the ct which I have referred was an old one and not designed to modern standards.

One last query. Can the driver on uk mainline see a red aspect(aspect cleared to yellow and train travelling at line speed) when the interlocking is up and working(excluding spad, axle counter restoration process etc). Signal going to red due to replacement joint less than 5.5 m.

Sorry for too many queries :-(
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#6
In general I would expect last wheel replacement for a GPL regardless of whether fitted with TPWS or the precise distance of the replacement joint. Whereas modern shunt signals often prove all tracks clear in route and even overlap, traditionally there was far less in terms of track controls and in some cases hardly any at all.

In "real shunting" the locomotive is often at the back end of the train and propelling the wagons and therefore the signal could be replaced well before the driver got close to the signal itself due to length of the train being pushed ahead of the loco. When there are track controls in the aspect, then the $40 presentation shows those tracks that are "non-replacing" and this is applied to all the track sections that are closer to the GPL than the maximum anticipated length of the propelled train.
Clearly if the signal aspect does not prove certain track circuits clear, then there is nothing to which to apply the $40. So obviously if the GPL proves no tracks at all, then no $40 entry!
Also if you are looking at an older style control table then the control may be shown rather differently and so whereas it may be "last wheel" the presentation can be very different.
Typically on the BR Western Region most GPLs would not proves track clear at all. However there would be "signal replacement" so that when a train uses the signal it reverts to danger and stays at danger until cancelled and reset by the signaller. Replacement conditions were specified by such annotations as #1, #1#4, #1#5, #1#4#5. These were used to signify 1st wheel replacement or various forms of delayed replacement (stick track occupied and cleared, berth track clear after berth track occupied etc.) Other practice also exist- for example L within a circle can signify "last wheel replacement". So just because the existing Control Table doesn't mention $40 this should not be interpreted as being first wheel- it may be that the information is just presented differently.--------------------------------------------------------------------------

Not sure I have understood your last question- ask again if I have misinterpreted.
UK mainline driver on 3 aspect signalled line travelling at maximum speed but approaching a signal at Yellow and thus expecting to apply brakes normally to be able to stop at next signal at red. We would not expect that the driver would see the signal revert to red as their train passes it (although a passenger looking out of the window of the first coach would do so). Indeed if there happened unusually to be two locomotives at front of train and a driver in each (unusual as generally the locos would be working in multiple with the 2nd one working by remote control as a slave of the leading loco) then the driver in the 2nd loco would see the reversion and because of their training would not be concerned as would understand the reasons and not be concerned.

The circumstances when a driver could be approaching a yellow and see it revert to red would include:
a) failure condition (loss of point detection, track failure, TPWS proving fault, communication fault, TFM failure, interlocking failure, loss of power supply etc),
b) emergency action by signaller deliberately to try to get the train stopped at first opportunity via emergency braking as soon a a driver becomes aware of aspect reversion,
c) signaller's error- inadvertently cancels the route a bit prematurely, cancels wrong signal by mistake, cancels signal as has reassessed the routing decision thinking that they can "get away with it as driver outside aspect sequence range but misjudges and has actually left it too late.
d) Signaller has already communicated with the driver and told them this will occur (e.g. train signalled to depart a platform but then not being ready to leave - train fault, lack of train crew, passenger taken ill etc) and recognising that it will be detained there for a while so needing to run other trains in the area in the meantime.

These are all ABNORMAL circumstances of course- generally a driver should never see the aspect of a signal change to a more restrictive one- unlike a road user who when approaching a green traffic light must always expect this to change via amber to red.


(06-08-2014, 05:17 PM)jenni.joseph9 Wrote: Thanks again for ur reply.

You are correct that the ct which I have referred was an old one and not designed to modern standards.

One last query. Can the driver on uk mainline see a red aspect(aspect cleared to yellow and train travelling at line speed) when the interlocking is up and working(excluding spad, axle counter restoration process etc). Signal going to red due to replacement joint less than 5.5 m.

Sorry for too many queries :-(
PJW
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#7
Hi,

Thanks alot for the detailed reply. Its clear now.

Thanks & Regards
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