2010 Q3 Track Circuit SCWO

[PJW]

An attempt for comments please

- not timed

2010 Mod1 q3 DAP

OK as far as it went, but somewhat too brief for 30 minutes (but reasonable if you had allocated 20 minutes as indeed candidates had to do in that year), though I note that you didn't actually time yourself.

Scenario 2 also requires some rail break /poor series bonding of jumpers etc. somewhere along the track between the train axle and AA relay end(or a poorish train shunt that means with two track feeds in parallel the remaining rail to rail voltage is such that AA relay does not actually drop- however this partially duplicates scenario 1) because a short on the track is still a short wherever the feed has come from.

Scenario 3.  Probably needed more explanation (similar to 2) why the train shunt would not still short out; think best to talk about traction return large currents producing a dc offset down the length of a long section, the fact that one rail is effectively earthed by virtue of the traction return etc.

It is of course a mod1 question so I wouldn’t have expected to have had to go into the technical reasons for interference in great detail in this module.  In fact II think that it would therefore have been better to have amalgamated your scenarios 2 &3 and choose a more different one for the third.  

For example you hadn’t included considerations of physical stagger between IBJs and non-compliance with minimum length of track sections so certain wheelbase / inter-bogie spacing vehicles could escape detection since axles not actually in a location that does short out the track circuit.  This would both avoid the detail (which does not seem very applicable to the module and I don’t think is your strong point either) and open up to be more inter-disciplinary; a track circuit that used to be OK might fail to detect a vehicle that had not previously had cause to be utilised in the area.

Distinct reasons would then be:
1. Train shunt ineffective at rail interface
2. Undesired voltage presented to track relay
3. Dimensional incompatibility between vehicles and infrastructure
Each could have had sub-bullets (first one: wheel-wheel resistance, weight distribution on vehicle, wheel profile, rusty rails, icy rails, leaf fall mulch, surface contamination such as coal dust, sand applied to improve adhesion etc.)


However, going with what you selected, I thought page 2 was fine but you could have include what could be done in the wider signalling system to mitigate the consequences. Slow to pick TPRs (equivalent in SSI track special) is standard.  When ac vane tracks needed to be immunised for Eurostar, the the VT1(SP) was invented to add more “slow to pickness” in the location.  In ARS areas in particular, the interlocking utilises “15 sec track bob protection” for route holding if next track not occupied.  For lines were directionality can be reversed, then the long section route locking demands all tracks clear for 15 seconds in addition to the usual sectional route release. In leaf fall areas the interlocking can be configured to add extra delay in aspect clearance after any track has just picked.  Control systems such as IECC give “track out of sequence” alarms to the signaller.  I am sure that you know all this but you didn’t include anything of this ilk.

Initially I was reading the last para as part of item 2, but then realised that you must have intended it to have been 3.

You seemed only to be addressing the question from the perspective of a new works scheme, pre-commissioning; this is unduly limiting the question and constraining the scope of your answer.  So if you had considered the situation that a commissioned track was then discovered to be failing wrong side, that gives you more to discuss about the potential mitigations.  Certain rolling stock types could be temporarily prohibited from the route, or perhaps trains normally consisting of just single vehicles (e.g. a class 150) would be permissible if working in multiple so that there was a longer formation/ more axles.  Perhaps the risk of the possible WSF could be adequately mitigated in the short term by issuing signaller's instructions to disable ARS, key points into the required lie when routing over them and / or implementing a form of “double block working”.  

I think you were answering too much from just a signal engineering design perspective and not displaying a more holistic railway understanding. More consideration of the broad spectrum of how to manage the associated risk would have both given you more material and would have better aligned the answer with module 1 syllabus. Even just making clear which of your proposals were addressing the likelihood and which the consequences of the risk would have helped give the examiner the impression that you were very conscious of the syllabus of the module in whose context the question was asked.

It wasn’t a bad answer; I think that it should have passed but certainly wasn’t one of your better ones. Perhaps for the first 12 marks you’d have got: 2+2+2 and for the next 8 you’d have got 2+2+2 making it 12/20 (when I added the separate scores felt these overall didn’t reflect the subjective feel of the answer and then I realised that question in 2010 was out of 20 rather than 25); primarily because too focussed on the technical and not enough “management of safety”.  However note that 2010 was the year when candidates expected to do three questions in an hour and thus had to answer each in 20 minutes; this proved virtually impossible so that is why module 1 finally became like the other written papers regarding the number of questions and time allowance.