Headway and signal spacing exercise

[onestrangeday]

Hi Signalling Professionals:

The attached files are my another try for the headway and signal spacing calculation for a mixed line railway network.

Please check my work, any comments or suggestions are highly appreciated.

[PJW]

This is an interesting scenario; rather more involved than is typical for IRSE module 2.
Given that the scenario has ore and wheat trains, I am guessing that the exercise might be one set by the Queensland University open leanning course; if this is indeed the case it might be inappropriate that I commented in detail.
However it is correct that the signalling would have to reflect all traffic permitted to use the relative portio of the layout and the stopping distance of the ore trains exceeds that of the passenger service and therefore must dictate the signal spacing. Indeed althoough from a headway perspective the wheat trains are all but irrelevant (versy sparse, only short "season") they are still relevant for braking distance considerations.

So in part b) when considering the 4 train/ hour, why did you use 700m as the signal spacing, when in part a) you correcty pointed out that the braking for the ore trains would require a signal spacing of at least 830m?

Your comment re the "workable headway" is valid, but I'd have expected the answer to have been based on one specific railway's practicesa as assumed by the candidate. However the distinction doesn't seem of significanc,e given that you state headway acheived is 92 seconds when the timetabled requirement is only 4 trains per hour and there appears not to be a required non-stop headway explicitly stated. Was there in fact a need to undertake these calculations at all?

As you have recognised it will be the 3 min stopping headway requirement that will dictate (but of course this should also have been undertaken with signals spaced appropriately for the ore trains...)

Hi Signalling Professionals:

The attached files are my another try for the headway and signal spacing calculation for a mixed line railway network.

Please check my work, any comments or suggestions are highly appreciated.

[onestrangeday]

Hi PJW:

Sure, no worries I understand what you have said, thanks for your comment and suggestion that it always helps me to learn alot. I think I know where I could make my improvements in order to reflect on the situation described in the exercise.

But I can see that it poses much of difficulty when you have mixed line railway network, since you have to take all the situation into account and it could lower the overall throughput for passenger trains. Is it usual to have mixed traffic implemented in railway network ? or there is no real answers.

This is an interesting scenario; rather more involved than is typical for IRSE module 2.
Given that the scenario has ore and wheat trains, I am guessing that the exercise might be one set by the Queensland University open leanning course; if this is indeed the case it might be inappropriate that I commented in detail.
However it is correct that the signalling would have to reflect all traffic permitted to use the relative portio of the layout and the stopping distance of the ore trains exceeds that of the passenger service and therefore must dictate the signal spacing. Indeed althoough from a headway perspective the wheat trains are all but irrelevant (versy sparse, only short "season") they are still relevant for braking distance considerations.

So in part b) when considering the 4 train/ hour, why did you use 700m as the signal spacing, when in part a) you correcty pointed out that the braking for the ore trains would require a signal spacing of at least 830m?

Your comment re the "workable headway" is valid, but I'd have expected the answer to have been based on one specific railway's practicesa as assumed by the candidate. However the distinction doesn't seem of significanc,e given that you state headway acheived is 92 seconds when the timetabled requirement is only 4 trains per hour and there appears not to be a required non-stop headway explicitly stated. Was there in fact a need to undertake these calculations at all?

As you have recognised it will be the 3 min stopping headway requirement that will dictate (but of course this should also have been undertaken with signals spaced appropriately for the ore trains...)

Hi Signalling Professionals:

The attached files are my another try for the headway and signal spacing calculation for a mixed line railway network.

Please check my work, any comments or suggestions are highly appreciated.

[PJW]

Indeed a mixed traffic railway is very different to a metro line with a dedicated fleet of identical trains where the signalling can be customised to suit their exact characteristics.

Timetabling on a line which has a variety of speeds of traffic is very important; capacity is lost between trains of different speeds, so a group of fast trains are often scheduled to follow each other, then a similar "flight" of slower trains, then a gap and then another flight of faster trains (the gap being the lost capacity and arranged such that the first of the fasts just catches up the last of the slows by the place where they can be overtaken such as a station with a passing loop or a divergence of lines where the different services are routed to different destinations.

Also the signal spacing relates to braking distance and therefore where this varies between different traffic types, then this cannot be optimised in the same way as it could be if only one variety. Setting differential speed restrictions does help in equalising the best signal spacing for different types. One of the advantages of ETCS is that the lineside signals can be eliminated and each particular train works out where it needs to brake, based upon the train's own characteristics, the speed it is actually at that moment travelling, forthcoming gradient etc; thus this element of the problem goes away, but the loss of capacity due to different speed profiles remains.

Mixed traffic railways are indeed very common; a century ago on UK mainlines the fastest trains could be travelling at up to 100 mph whereas there were freight trains without any effective braking system constrained to trundle around at 20mph. Most lines had been built in the age in which the fastest form of transport had been a person on horseback on one of the few turnpike roads that were not significantly better to those built by the Romans some 1800 years previously; hence even 20mph was seen as quite a fair turn of speed!

Nowadays it is unusual for trains not to be able to travel at least at 50mph, yet these do still need to share tracks with express trains regularly travelling at 125mph. It was the French who led the way in building new fast lines dedicated to express passenger trains and thus permitting segregation with the slower trains remaining on the "classic lines". Europe has been building more of this modern railway engineered for speeds of up to 350kph and far less concerned re very shallow gradients (reflecting the far greater power of electric traction compared to steam locomotives); in the UK we only have the link line from London St Pancras International to the Channel Tunnel [which only slightly tongue-in-cheek is referred to by the French as "Trein Grande Vitesse Nord Plus"]. There is currently a strong possibility that the first section of a line (High Speed 2) to Scotland (but only initially as far as the outskirts of Birmingham) being built in the next few years IF there is money for it, IF it is considered worth the damage it will inflict to the Chilterns Area of Outstanding Natural Beauty.......

Hi PJW:

Sure, no worries I understand what you have said, thanks for your comment and suggestion that it always helps me to learn alot. I think I know where I could make my improvements in order to reflect on the situation described in the exercise.

But I can see that it poses much of difficulty when you have mixed line railway network, since you have to take all the situation into account and it could lower the overall throughput for passenger trains. Is it usual to have mixed traffic implemented in railway network ? or there is no real answers.

[onestrangeday]

Hi PJW:

thanks for your information on mixed traffic railway. It would be quite interesting to see that if 'HS2' is actually implemented in UK in coming years.

Indeed a mixed traffic railway is very different to a metro line with a dedicated fleet of identical trains where the signalling can be customised to suit their exact characteristics.

Timetabling on a line which has a variety of speeds of traffic is very important; capacity is lost between trains of different speeds, so a group of fast trains are often scheduled to follow each other, then a similar "flight" of slower trains, then a gap and then another flight of faster trains (the gap being the lost capacity and arranged such that the first of the fasts just catches up the last of the slows by the place where they can be overtaken such as a station with a passing loop or a divergence of lines where the different services are routed to different destinations.

Also the signal spacing relates to braking distance and therefore where this varies between different traffic types, then this cannot be optimised in the same way as it could be if only one variety. Setting differential speed restrictions does help in equalising the best signal spacing for different types. One of the advantages of ETCS is that the lineside signals can be eliminated and each particular train works out where it needs to brake, based upon the train's own characteristics, the speed it is actually at that moment travelling, forthcoming gradient etc; thus this element of the problem goes away, but the loss of capacity due to different speed profiles remains.

Mixed traffic railways are indeed very common; a century ago on UK mainlines the fastest trains could be travelling at up to 100 mph whereas there were freight trains without any effective braking system constrained to trundle around at 20mph. Most lines had been built in the age in which the fastest form of transport had been a person on horseback on one of the few turnpike roads that were not significantly better to those built by the Romans some 1800 years previously; hence even 20mph was seen as quite a fair turn of speed!

Nowadays it is unusual for trains not to be able to travel at least at 50mph, yet these do still need to share tracks with express trains regularly travelling at 125mph. It was the French who led the way in building new fast lines dedicated to express passenger trains and thus permitting segregation with the slower trains remaining on the "classic lines". Europe has been building more of this modern railway engineered for speeds of up to 350kph and far less concerned re very shallow gradients (reflecting the far greater power of electric traction compared to steam locomotives); in the UK we only have the link line from London St Pancras International to the Channel Tunnel [which only slightly tongue-in-cheek is referred to by the French as "Trein Grande Vitesse Nord Plus"]. There is currently a strong possibility that the first section of a line (High Speed 2) to Scotland (but only initially as far as the outskirts of Birmingham) being built in the next few years IF there is money for it, IF it is considered worth the damage it will inflict to the Chilterns Area of Outstanding Natural Beauty.......

Hi PJW:

Sure, no worries I understand what you have said, thanks for your comment and suggestion that it always helps me to learn alot. I think I know where I could make my improvements in order to reflect on the situation described in the exercise.

But I can see that it poses much of difficulty when you have mixed line railway network, since you have to take all the situation into account and it could lower the overall throughput for passenger trains. Is it usual to have mixed traffic implemented in railway network ? or there is no real answers.