Signal Gantry Spacing

[dangie]

From the novice again so please be gentle.

How are the distances between track sections (signals) worked out?

For example, at Rugeley Trent Valley on the WCML there is a signal gantry for the down line at the north end of the platforms. There is a signal gantry for the up line at the south end of the platforms. A distance between them of approx one hundred metres or so. Why couldn't the track sections be worked out so all signals could use the same gantry? A little farther up the line towards Colwich the up/down line signals are even closer together. Surely one gantry across all lines could carry both sets of signals.

Hope this makes sense. Thank you.

 

[The Planner]

Sighting and braking distances will play a good part in it.

 

[snowball]

I Imagine it's preferable operationally to have signals sited so that a train waiting at a platform has a signal just in front of it.

Very often there are crossovers or junctions soon after the platform and it's good to have a signal just before a junction (while complying with required overlap distances etc).

Braking distances and sighting considerations impose constraints on where signals can be but are not the only factor.

 

[MarkyT]

From the novice again so please be gentle.

How are the distances between track sections (signals) worked out?

For example, at Rugeley Trent Valley on the WCML there is a signal gantry for the down line at the north end of the platforms. There is a signal gantry for the up line at the south end of the platforms. A distance between them of approx one hundred metres or so. Why couldn't the track sections be worked out so all signals could use the same gantry? A little farther up the line towards Colwich the up/down line signals are even closer together. Surely one gantry across all lines could carry both sets of signals.

Hope this makes sense. Thank you.

Signals are spaced according to braking distance, which is looked up in official group standard tables that show the minimum for appropriate types of train, the speed on approach and the average gradient from signal to signal. In 3-aspect areas each signal is no less than one standard braking distance from the next applying in the same direction. With a 4-aspect section the signals are one half a braking distance apart. The braking distance is only a guideline minimum and might be varied upward to about 150% of the minimum with little difficulty, but excessive distances above this or very irregular distances would need risk assessing together with other factors such as visibility. Around stations and junctions signals are more precisely constrained by the positions of points and platforms. Trains often need to be able to stand at a red signal clear of a junction so other conflicting trains can pass in front. Such signals 'protecting' a junction are often placed a standard overlap distance (typically 200m) from the junction clearance point or that might be reduced for a lower speed approach. Signal positions at your Rugely example would be the design engineer's best compromise between the above factors together with the important constraint of good visibilty.

 

[dangie]

Thanks everyone. I knew there'd be an answer.

 

[455driver]

Sighting and braking distances will play a good part in it.

Hence why, on the down Pompey direct approaching Havant jn, we have a 3 mile long straight with a signal 300 yards around the corner at the end of it, they have installed a banner repeater at the end of the straight though! :roll:

If they had put the signal where the banner was it would have been perfect but no it has to go around the corner!

 

[Tomnick]

I understand the frustration - but those extra 300yds would surely increase the junction margins by anything up to a minute, decreasing capacity through the junction etc.?

 

[The Planner]

How much read through is there on that straight? must be two signal sections?

 

[455driver]

I understand the frustration - but those extra 300yds would surely increase the junction margins by anything up to a minute, decreasing capacity through the junction etc.?

If we had a proper view of the signal on approach then we would actually be quicker over the junction.

We nearly always get stopped at it anyway (a Southern service is due across 2 minutes before us), it is a 40mph jn and we might manage to get up to 20mph from a standing start and setting off on a yellow wont even get us into Havant as there is another signal between the junction and station.

It really doesnt make sense.

Dont get me started on the signalling between Portsmouth & Southsea and Portsmouth Harbour!
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How much read through is there on that straight? must be two signal sections?

It is 2 sections but the risks would be minimal as both signals are normal (not LED heads), I can think of several locations where you have 3 or even 4 signals in view.

 

[Tomnick]

Ah, but it'd take you longer to clear the junction if you're starting from 300yds further back! That'll affect the junction margins in turn, though that'd be more relevant to a Down train towards Brighton crossing in front of an Up train from Waterloo than one Down train following another. Probably only half a minute or so, but even adding that much into a schedule appears to upset the TOCs greatly.

 

[The Planner]

Junction margin should be based on seeing a green, not stopping unless you have shoved a load of pathing time in.

 

[Tomnick]

Interesting thought - if there is a load of pathing allowance approaching a junction (such that it'll almost certainly have to come to a stand), is the train then timed from a stand as opposed to the usual section times assuming it's running through? That's the scenario I was thinking about here!

 

[MarkyT]

Junction margin should be based on seeing a green, not stopping unless you have shoved a load of pathing time in.

Remembering that speed reduces significantly at Havant and virtually everything stops there, this signal like many similar examples protecting a junction is placed as close to the junction as possible, usually a standard overlap as in this example. A train could approach such a signal at red in planned or perturbed conditions. Either way with the 'holding point' brought in as close as possible, when the signal does clear the subsequent movement into the platform takes the least possible time, also blocking the junction for the shortest possible time.

 

[455driver]

Junction margin should be based on seeing a green, not stopping unless you have shoved a load of pathing time in.

Once i got used to which trains would normally get stopped I would just let the train roll all the way from Buriton tunnel and would then normally get a green.

 

[The Planner]

Interesting thought - if there is a load of pathing allowance approaching a junction (such that it'll almost certainly have to come to a stand), is the train then timed from a stand as opposed to the usual section times assuming it's running through? That's the scenario I was thinking about here!

No, it is a personal bugbear of mine too, the train will be assumed to be going linespeed after the pathing time. I think some planning areas do cover that eventuality in the planning rules (Western IIRC without checking) but most don't so you are losing straight away.
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Remembering that speed reduces significantly at Havant and virtually everything stops there, this signal like many similar examples protecting a junction is placed as close to the junction as possible, usually a standard overlap as in this example. A train could approach such a signal at red in planned or perturbed conditions. Either way with the 'holding point' brought in as close as possible, when the signal does clear the subsequent movement into the platform takes the least possible time, also blocking the junction for the shortest possible time.

Makes no difference in planning, you work to the prescribed junction margin which is based off the best available aspect possible (ie: a green), if not you chuck pathing time in as above to achieve that margin. Perturbed conditions don't factor in our job.