(04-05-2016, 10:11 PM)dorothy.pipet Wrote: Another attempt for comments please
I didn't think this was one of your better answers.
This was partly because I think that assuming UK mainline with Great Western ATP was not a good starting place for the following reasons:
1. Don't think of this as "simple ATP". It is certainly old, but it does provide continuous speed supervision and controls the train down to a low release speed when nearing the end of movement authority. It is probably "too good" and could therefore cramp your answer later on.
2. It is quite bulky and needs quite a lot of interfacing to the vehicle; not really something that one would want to fit to a large number of trams, even if could still obtain.
I think that the examiners were thinking more of a train-stop type train protection with TPWS-like functionality although probably something like Indusi which would be fitted to the tram but not the mainline trains. I know that in the various signal overrun standards it is emphasised that in that context AWS and TPWS do not constitute "train protection", but it is my contention that they would do within the context of this question and for IRSE exam more generally.
In general I didn't feel that you had made enough of the tramway-like nature of the light rail trains; in fact you tended to refer to them just as "passenger trains" and therefore there is the risk that the examiners may feel that you hadn't read the question carefully enough to fully appreciate the scenario described.
For part i) you only really had four hazards; by labelling 1,2 & 3,4 where you did, it did unfortunately rather draw attention to the accident rather than the requested hazard; if it had not been for 5 & 6 I think I'd have misread your answer and then criticised for that reason.
For the exceedance of movement authority and overspeeding and it was as you stated only the consequences which might be exacerbated by the lower crash resistance of a tram type vehicle compared with a heavy rail passenger.
Your items 5 and 6 were better (but I think you should have made more of 6 in less dramatic circumstance than exacerbating a separate accident).
a) Coal could fall from the top of a vehicle, particularly if loaded too high / unevenly compared with hopper sides and if condition of the track none-to-good.
b) Coal might also have spilt from the hopper loading operation and become lodged elsewhere on the vehicle, only to become dislodged again sometime en route.
c) The hopper bottom doors may not have been properly secured shut after the previous discharge operation (or indeed it is not unknown for vandals / thieves deliberately to release some coal for their own use illegitimately when train waiting in a loop or at a junction etc. and they tend not to be too concerned about securing again afterwards!).
I think that you should definitely have concentrated on the hazard that track circuits may show clear when occupied. The trams would be light and short with few wheels in contact with the rails. Likely to have a different wheel profile so may run on part of the rail head different from that utilised by the heavy freight. These trains may well be contaminating the are with coal dust and if rail conditions aren't good for adhesion the locomotive could be laying down quite a lot of sand. The question therefore seems to be setting up a recipe for loss of train shunt, the result of which could be that the coal trains which travel at higher maximum speed than the trams which presumably may stop at a station en route. Therefore there looks like an accident scenario in which a coal train which is running on clear aspects then piles into a more slowly moving tram.
In leaf fall season there could be a different hazard leading to a similar rear end collision accident, should adhesion conditions mean that the freight driver is unable to control their train as usual.
Could also have considered whether the tramlike vehicles ever operate on line-of-sight on part of their journey once left the mainline. The drivers therefore have to follow different rules in various operating modes and may potentially become confused between the different scenarios, particularly in failure conditions and other unusual circumstances.
If authorised past a signal at danger prepared to stop short of any obstruction, the driver mind-set may be that it is like driving in a street section which may be a usual operation elsewhere. Hence habit may kick in, but instead of the streets being well lit and the tram ahead being well lit, when on the mainline it might be very dark and the only indication that they are imminently to encounter a stationary freight train is a sole small tail lamp.
The greater diversity of hazards would then have opened up the question and given you more to discuss in the way of mitigation; axle counters rather than track circuits perhaps?
Collision bars / "Cow catchers" on the trams would have been worth suggesting re lowering derailment risk if encountering obstructions.
For misrouting, if the train protection was unique to the trams, then it would still be active if route 3A set, hence mitigate against the tramlike vehicle being routed to inappropriate railway.
Probably ought to have made more re whether it would be possible to signal the freight onto and off from the joint line in a single operation (i.e. 21 requiring the whole section clear and route 3A set, whilst then allowing 11 to clear once freight had passed the overlap of 9). The light rail PEAK frequency is 10 mins; presumably since there are only 3 freights a day these can be scheduled not to travel on the line during the actual peak periods. If travelling at 100km/h continuously then the 10km would then only take some 6 minutes after the last passenger, and the next passenger able to closely follow the freight it is looking possible. However the reality is that the freight would surely be starting from rest, likely to have a very slow acceleration and potentially may need to slow again when the lines separate depending on turnout speed.
Does seem a good option though. Let us assume that off-peak the tram service is every 15 minutes and there is a need to squeeze the freight through between two services. I guess that this would mean the second of the pair would be arriving at signal 11 when the earlier one was just vacating the shared line (travels only at 80km/h max and may perhaps stop in the section a couple of times so perhaps takes 10 minutes to get entirely through). So in order to get the freight a path then this would be sent instead whilst the following passenger is held at signal 11. The freight that would then lumber out and then need to continue as far as to get its rear vehicle clear of signal 9's overlap before 11 could clear to yellow for the tram to begin to move. This would probably cause delay the following tram by around 5 minutes but once on the line would be free to operate normally as the freight should then show it a clean pair of heels given that the tram is likely to be stopping soon anyway and its maximum speed is more limited. Inconvenient but it is only going to happen 3 times a day and if that is the cost of operating this new service over the existing freight tracks (sounds very 1980s Newcastle on Tyne to me) in a safe manner, so be it. In fact we could enforce by putting a single axle counter section overlaying the whole common section of the line from 11 to the clearance points beyond the diverging junction at the far end and prove this in the signal from the freight line, thus being confident that the entire section is free of trams and also that the route is set to the correct line beyond. My assumption is that the line was a historic freight line with TC train detection and providing a single overlay axle counter is far cheaper than reequipping the line as a whole and that the risks associated with wsf of train detection are considered low enough if the vehicle behind is also tram vehicle which is light and has relatively good brakes.
You spoke about "double red" for the freight presumably to mitigate against the risk of derailing at the trap points at the convergence- perhaps it might have been practicable to give a longer length of available overrun. Without any form of train protection, some of the value of double red is not achieved.
I didn't see any part of the answer addressing iii)- assume you ran out of time but that meant 20% of the marks were unavailable to you
Therefore I felt overall that this answer would have been about a Pass or just under, which is a shame given the amount of content.
