13-06-2014, 01:13 PM
Headway Calculation
The headway calculation is very well performed, one of the best I’ve seen.
Minor criticism – no braking distance calculated for 40 Kmh or 100 kmh.
It might be worthwhile to draw a simple diagram of two trains at the headway critical moment, to justify the equations used for 3 and 4 aspect headway time.
Although 1.33 was used in the past, 1.5 is usually used today as the maximum signal braking distance - adopting this more relaxed limit will always make your task of signalling the layout easier.
The stopping calculation is very well presented and concisely argued - this is essential in the limited time available.
The final graph is a good illustration. One error in the dimensioning is that you have marked 118.4 s (non stop HT) in place of 215s (stopping HT).
The layout
You need to state which network rules you have used - I assume “UK Main Line” ?
Query signal 101 is positioned as for a 3 aspect signal, but has a 4 aspect profile.
You would benefit from studying the methods which can be used to mitigate underbraked 3 aspect signals and to achieve transition 3 to 4 aspect. The two problems and their solutions are very similar. One solution involves applying MAR to the previous signal, when the underbraked signal is ON; the other method uses an additional distant signal for the underbraked signal. These methods would let you manage the transition from 3 to 4 aspect on both lines in this layout.
The provision of ROL at signal 103 is inappropriate – only suitable where there is a convergence (or very occasionally opposing overlap limit) within the full overlap area. Remember that with 4 aspect signalling, the basic requirement is to have (1- 1.5 *) braking distance from YY to R. This means that it’s OK to have greater than 0.5 braking distance in one signal section, provided it’s made up for in the neighbouring sections. In your case, positioning signal 103 at dimension 0300 would give 1850m to signal 111- still less than your calculated 2015m maximum.
The down goods should probably only require 3 aspect signalling, due to the lower braking distance at 100 kmh.
107 signal requires an overlap towards the spur line – suggested position 1700m to include the diamond.
Diamond crossing Down Goods / spur line:
• Is fixed - you have added “switch diamond” symbols.
• Will require trap points on both sides, to protect passenger spur line movements from Goods incursions.
Down siding also requires a trap point for the same reason.
Spur Line: 4 aspect signalling is inappropriate (and overbraked for the low speed). Would be better to use one train working controlled by track circuit and/ or 2 aspect.
Signals 111/ 109 should be placed at 2100m to give full overlaps to the points.
Signal 109:
• MAR not required as both routes have same speed.
• Standard indicator more suitable
Signal 113 would be more appropriate as a shunt signal (run round moves only)
Signal 115 would be more appropriate as 3 aspect with standard indicator.
Route 115B appears to take the Up Branch and “skip” signal 151 - is this intended?, if so, a down direction signal required alongside 151 on the Up branch.
Ongoing signals 117 & 119 are unconstrained by layout features, so the signal spacing could be extended within the range calculated to reduce cost.
Up Siding E:
• No method has been provided to enter siding - requires down direction shunt signal at 1040m.
• Separate track circuit GA not required.
• Siding track circuit not named (but not required either)
Signal 122 should be placed at position 2850m to provide full overlap clear of points. See previous comments about ROL and 3 to 4 aspect transitions.
Note that platform end signals are not obligatory to instruct making a station stop - they often fall in this place to protect pointwork beyond the station only. You can therefore omit signals 110/ 108, and reposition 106/ 104 to position 0800m. this would then give correct 2050m braking from 122 (YY), via 116 (Y) to 106 ®
Subsidiary aspects/ call-on routes not required on 114/ 116 signals:
• Platform sharing not required by the spec.
• A goods train from 114 should not be called-on behind a passenger train at 110.
• The main class routes can be used for the “shunting” (but not necessarily shunt class) run-round movements.
Signal 151 inappropriately positioned with points in overlap – could reposition at 2950m to give 400m standage from Up Main and 100m reduced overlap – mitigated by 40kmh approach speed.
Signals 154 and 153 both inappropriately positioned to stop trains in the tunnel (also very difficult to sight). There’s actually no need for either of these signals:
• Since the “junctions” which they protect give no choice of route, signals are not necessary – any speed restriction through the points can be managed using permanent speed restriction arrangements.
• It is operationally inconvenient to stop trains in the single line section
• A better solution would be for 151 route to pass all the way to 155, and 156 all the way to 152
Signal 156 inappropriately positioned - see previous ROL comment.
Signal 152 better to use standard indicator.
Limit of shunt required on up branch at approx 2100m to terminate running-round locomotives.
Call-on class route required at 152 signal to allow run-round locomotive to rejoin train at 114.
Trap points required in Up Goods at 2450m.
Route tables could be done faster by only writing column headings once (“these are my column headings for all route tables”). Could reasonably omit for simpler signals such as 302, 301 etc.
Good provision of point positions and numbering.
Good provision of train detection and numbering.
Overall
Although I appear to have made a lot of comments, they are mostly “fine detail” in nature. The layout has been comprehensively signalled in accordance with principles and the spec. Coupled with the good headway calculation and points/ train detection provision, I think that this response would comfortably achieve a good “pass” grade. Not a lot of improvement would be needed for a “credit” to be feasible.
The headway calculation is very well performed, one of the best I’ve seen.
Minor criticism – no braking distance calculated for 40 Kmh or 100 kmh.
It might be worthwhile to draw a simple diagram of two trains at the headway critical moment, to justify the equations used for 3 and 4 aspect headway time.
Although 1.33 was used in the past, 1.5 is usually used today as the maximum signal braking distance - adopting this more relaxed limit will always make your task of signalling the layout easier.
The stopping calculation is very well presented and concisely argued - this is essential in the limited time available.
The final graph is a good illustration. One error in the dimensioning is that you have marked 118.4 s (non stop HT) in place of 215s (stopping HT).
The layout
You need to state which network rules you have used - I assume “UK Main Line” ?
Query signal 101 is positioned as for a 3 aspect signal, but has a 4 aspect profile.
You would benefit from studying the methods which can be used to mitigate underbraked 3 aspect signals and to achieve transition 3 to 4 aspect. The two problems and their solutions are very similar. One solution involves applying MAR to the previous signal, when the underbraked signal is ON; the other method uses an additional distant signal for the underbraked signal. These methods would let you manage the transition from 3 to 4 aspect on both lines in this layout.
The provision of ROL at signal 103 is inappropriate – only suitable where there is a convergence (or very occasionally opposing overlap limit) within the full overlap area. Remember that with 4 aspect signalling, the basic requirement is to have (1- 1.5 *) braking distance from YY to R. This means that it’s OK to have greater than 0.5 braking distance in one signal section, provided it’s made up for in the neighbouring sections. In your case, positioning signal 103 at dimension 0300 would give 1850m to signal 111- still less than your calculated 2015m maximum.
The down goods should probably only require 3 aspect signalling, due to the lower braking distance at 100 kmh.
107 signal requires an overlap towards the spur line – suggested position 1700m to include the diamond.
Diamond crossing Down Goods / spur line:
• Is fixed - you have added “switch diamond” symbols.
• Will require trap points on both sides, to protect passenger spur line movements from Goods incursions.
Down siding also requires a trap point for the same reason.
Spur Line: 4 aspect signalling is inappropriate (and overbraked for the low speed). Would be better to use one train working controlled by track circuit and/ or 2 aspect.
Signals 111/ 109 should be placed at 2100m to give full overlaps to the points.
Signal 109:
• MAR not required as both routes have same speed.
• Standard indicator more suitable
Signal 113 would be more appropriate as a shunt signal (run round moves only)
Signal 115 would be more appropriate as 3 aspect with standard indicator.
Route 115B appears to take the Up Branch and “skip” signal 151 - is this intended?, if so, a down direction signal required alongside 151 on the Up branch.
Ongoing signals 117 & 119 are unconstrained by layout features, so the signal spacing could be extended within the range calculated to reduce cost.
Up Siding E:
• No method has been provided to enter siding - requires down direction shunt signal at 1040m.
• Separate track circuit GA not required.
• Siding track circuit not named (but not required either)
Signal 122 should be placed at position 2850m to provide full overlap clear of points. See previous comments about ROL and 3 to 4 aspect transitions.
Note that platform end signals are not obligatory to instruct making a station stop - they often fall in this place to protect pointwork beyond the station only. You can therefore omit signals 110/ 108, and reposition 106/ 104 to position 0800m. this would then give correct 2050m braking from 122 (YY), via 116 (Y) to 106 ®
Subsidiary aspects/ call-on routes not required on 114/ 116 signals:
• Platform sharing not required by the spec.
• A goods train from 114 should not be called-on behind a passenger train at 110.
• The main class routes can be used for the “shunting” (but not necessarily shunt class) run-round movements.
Signal 151 inappropriately positioned with points in overlap – could reposition at 2950m to give 400m standage from Up Main and 100m reduced overlap – mitigated by 40kmh approach speed.
Signals 154 and 153 both inappropriately positioned to stop trains in the tunnel (also very difficult to sight). There’s actually no need for either of these signals:
• Since the “junctions” which they protect give no choice of route, signals are not necessary – any speed restriction through the points can be managed using permanent speed restriction arrangements.
• It is operationally inconvenient to stop trains in the single line section
• A better solution would be for 151 route to pass all the way to 155, and 156 all the way to 152
Signal 156 inappropriately positioned - see previous ROL comment.
Signal 152 better to use standard indicator.
Limit of shunt required on up branch at approx 2100m to terminate running-round locomotives.
Call-on class route required at 152 signal to allow run-round locomotive to rejoin train at 114.
Trap points required in Up Goods at 2450m.
Route tables could be done faster by only writing column headings once (“these are my column headings for all route tables”). Could reasonably omit for simpler signals such as 302, 301 etc.
Good provision of point positions and numbering.
Good provision of train detection and numbering.
Overall
Although I appear to have made a lot of comments, they are mostly “fine detail” in nature. The layout has been comprehensively signalled in accordance with principles and the spec. Coupled with the good headway calculation and points/ train detection provision, I think that this response would comfortably achieve a good “pass” grade. Not a lot of improvement would be needed for a “credit” to be feasible.