Look at what is shown at the left-hand end of the layout:
a) Between B and G, determine the
best theoretical headway for both 3 and 4 aspect signalling at uniform minimum signal spacing and non stop train speed of 140 km/h. Explain how the results of the headway calculation determines your choice of signalling system.
So for a), you do not worry
initially about the headway required by the train service; you are just saying what is the best that can be achieved by 3 aspects and the best that can be achieved by 4 aspects.
However to answer the last sentence you
do need to compare these values with what is needed for the layout. You are indeed correct that in 2013 paper there is not a specified minimum time between following trains, but what you need to do is look ahead to section c). This tells you the traffic requirements on various portions of the layout. You need to make an assessment of the traffic needs yourself from this information.
Look at the section C to D and add up how many trains are to use it in a one hour period. Basically it looks like 5 passenger trains plus 2 freights every hour and once in the day the empty passenger stock movement. Simplistically you could ad up and say that is 8 trains per hour and therefore a 7 minute headway would suffice. This is obviously
incorrect because there is a mixture of different sorts of trains- the speed of the nonstop trains quite a bit faster than the freights and indeed the stopping passengers will traverse the section even slower as they will be braking, dwelling and accelerating again after the station. Therefore these trains "eat up" more capacity than the equivalent number of fast trains would have.
In addition the 8 trains are not likely to be timetabled to present themselves at time separations equally spaced during that hour and it is necessary that the signalling can provide for one train to follow another rather more closely- some gaps between trains will be greater than the nominal 7 minute average hence other gaps will be smaller; the signalling must be designed to cope with the smallest separation likely to arise. This is before we take intro account that the signalling needs to have an element of contingency built in to make allowance for the fact that in reality time keeping isn't perfect and certain trains sometimes may be a little late- we wouldn't want one delayed train to have knock on effects to lots of other services so there must be an element of contingency for such perturbations.
Hence it is time to
state an assumption based on experience and say that, having considered the above factors, you have determined that the signalling should be capable of providing double that number of train paths and therefore ought to be designed to deliver 3.5 minute headway.
Remember it is
not the precise number that you should worry about, and others might have argued for a 2.5 minute headway whereas others for a 4.5 minute. None of these are necessarily wrong. The examiners won't quibble; without knowing the timetabling of the various train services the smallest interval can't be known. It is the factors which you know you ought to take into account, the explanation of why and the logical process used which is more important. Ultimately you make "an educated guess" at a number you justify and then use that number to answer that last part of question a) andbefore finally using the solution when placing signals on the layout diagram.
b) Determine, graphically or by calculation, the
best headway for a non stopping train from B to G ..... under clear signals, following the departure of a stopping train from the Down Loop at Station C to the Down South Branch at Junction D.
c) Train running requirements in each direction:
Passenger: between B and G (non-stopping) 2 per hour
Passenger: between B and G (stopping) 2 per hour
Passenger: between A and C (Terminating at C) 1 per hour
Passenger: between A and E 1 per hour
Passenger: Running empty between Down Sidings at C and B 1 per day
Freight: between A and G 1 per hour
Freight: between B and E 1 per hour
Freight: from G to A calling at the Private Siding at F 1 per week
[/b]
(20-02-2014, 06:59 AM)sai.kish55@gmail.com Wrote: Dear all,
please find attached 2013 question paper and layout 1,
please clarify in layout 1 there are no required headway times(unlike previous year layouts ) then how we will decide which is best signalling(3 or 4 aspects) for the layout based on our calculations ?
Regards
Sai kishore Polana
