Question About Loops

[Mordac]

Looking at the sectional appendix for the WCML between Gretna and Carstairs, I noticed that a few, but not all, loops require through trains not using them to reduce to 95 or even 90 between the two ends of the loop. Now some other ones don't require any speed reductions for through trains, so is there any particular reason why these restrictions are there? Is it something to do with the points, or what? It seems like conversing all loops to allow through trains to go at normal speed would be a very cheap way of reducing journey times, so I'm guessing there must be something I'm missing?

NB: As a couple of examples, the loop at Quintishill requires reduction to 95 while having EPS speeds of 125 or 120 on either side, while the loop at Beattock doesn't require any changes.

 

[theageofthetra]

I would have thought it may be to do with signal sighting if a train was in the loop?

 

[najaB]

As a couple of examples, the loop at Quintishill requires reduction to 95 while having EPS speeds of 125 or 120 on either side, while the loop at Beattock doesn't require any changes.

Well, there is a curve near Quintishill so that may factor into it as well.

 

[matchmaker]

I'd slow down at Quintinshill anyway - just in case...

 

[scott118]

Loop speeds (excluding ESR/TSR's..) can only be dictated by signal sighting and the point length. AFAIK.

 

[Joseph_Locke]

Just coincidence in these examples. Just to the south, both loops at Oubeck are 110/125EPS on the main line, as are all the loops At Carnforth, Hartford, etc.

To have 125mph through these loops would require 125mph turnouts (you can't allow tilt through the turnout route) which we don't (yet) have ... come back the RT60J, all is forgiven ...

Loop speeds (excluding ESR/TSR's..) can only be dictated by signal sighting and the point length. AFAIK.

And how far it is to the loop exit signal ...

 

[LNW-GW Joint]

I'd slow down at Quintinshill anyway - just in case...

I don't really see the point of the Quintinshill loop, with a 4-track railway 2-4 miles to the south.
It must be a relic of the Cal/G&SW days and congestion at Gretna Jn.

 

[AndrewE]

Just coincidence in these examples. Just to the south, both loops at Oubeck are 110/125EPS on the main line, as are all the loops At Carnforth, Hartford, etc.

I think the question is why are the main line speeds reduced for the length of the loop. Could it be a typo in the sectional Appendix? No-one would expect full linespeed through the loops (unless it's something odd like a central tidal flow track.)

 

[Mordac]

I think the question is why are the main line speeds reduced for the length of the loop. Could it be a typo in the sectional Appendix? No-one would expect full linespeed through the loops (unless it's something odd like a central tidal flow track.)

That's exactly what I meant, yes! I was just struck by it because the speed reduction begins just before the loop does, and ends just after the loop ends.

 

[furnessvale]

That's exactly what I meant, yes! I was just struck by it because the speed reduction begins just before the loop does, and ends just after the loop ends.

Could it be pure coincidence?

IMO, if I was looking to put a loop anywhere, one consideration would be a location, eg a curve, where all trains on the main line were having to slow anyway.

 

[scott118]

dictated by signal sighting and the point length.

And how far it is to the loop exit signal ...

Are they not the same thing?

 

[Trog]

Probably usually a co-incidence, as the only reason I can think of for there to be a restriction on the main running line coincident with a loop would be where the entrance and exit S&C is on a curve. As the rules regarding the use of cant / deficiency are tighter for S&C than they are for plain line.

 

[Joseph_Locke]

Are they not the same thing?

Nope. For a short loop you may have to be braking for the exit signal to such a degree that there is no point having a fast entry speed - nothing to do with sighting the signal or the p-way speed on the turnout.

For instance, under normal braking a 775m freight stops from 50mph in its own length, so if the loop isn't much longer than 800m then there's no point having a faster entry turnout than 50mph. The signalling system has to provide braking distance to this signal anyway.

 

[The Planner]

40mph is the expected turnout speed to loops now with a flashing yellow approach if possible.

 

[scott118]

Nope. For a short loop you may have to be braking for the exit signal to such a degree that there is no point having a fast entry speed - nothing to do with sighting the signal or the p-way speed on the turnout.

For instance, under normal braking a 775m freight stops from 50mph in its own length, so if the loop isn't much longer than 800m then there's no point having a faster entry turnout than 50mph. The signalling system has to provide braking distance to this signal anyway.

Luckily, my route knowledge will already have myself, prepared...regardless of loop length, point lead/turnout length and signal sightings. Thanks for explaining it better than i could though, to others that are reading this thread chap...

 

[driver9000]

Quintinstill loops are on a curve while Beattock loops are on a straight, Beattock Summit is 90mph with 95mph either side of it. The same at Abington loops which are 90mph on the main line with 95mph to the south and 105mph to the north again on a curve.
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I don't really see the point of the Quintinshill loop, with a 4-track railway 2-4 miles to the south.
It must be a relic of the Cal/G&SW days and congestion at Gretna Jn.

The 4 track section I think you mean is the Up/Down Main and access lines for Kingmoor between Floriston and Harker. The 4 track main line ends at DRS Kingmoor depot when leaving Carlisle station.

 

[AndrewE]

Probably usually a co-incidence, as the only reason I can think of for there to be a restriction on the main running line coincident with a loop would be where the entrance and exit S&C is on a curve. As the rules regarding the use of cant / deficiency are tighter for S&C than they are for plain line.

This seems the most likely explanation then, thanks for that.