What can you tell me about dynamic loops?

[whole diary]

[grahame]

What can you tell me about dynamic loops? - I guess the subject line says it!

If I want to have a train that's running every hour from (say) Windowchester to Yeodorf, and a train running every hour from Yeodorf to Windowchester passing it on the way ... both clock face, how long a double track section would I need within an otherwise-single line to allow the trains to pass at line speed between stations?   How many track circuit sections / signals / berths would I need in the double track dynamic loop?   Would it be easy to signal the loop so that it could also be used to have one train overtake another (e.g. passenger pass a freight)?

Looking at the Open Train Times map for the South Western line to Exeter - http://www.opentraintimes.com/maps/signalling/westofengland - I see a bewildering set of different signalling arrangements and would really appreciate a bit more of an understanding which I know some members can help me with ... the Wilts, Somerset and Weymouth has a couple of pinch points where there are aspirations 

Windowchester and Yeodorf are about 20 miles / 35 minutes by train apart at present, with one of the reasons for it taking so long being the need to stop and go through a palaver with tokens at Spinster Oldville.

[SandTEngineer]

Axminster dynamic loop is about three miles long.  At that length its difficult to run on clear signals in both directions so its better if one, or both trains stop at a station on the loop.  In the past I have done studies for dynamic loops and they don't really work if trains have to pass non-stop and at linespeed unless the loop is about 10 miles long.

When we were looking at dynamic loops on the Salisbury - Exeter line, in my Railtrack Southern days way back in 1997, the best options were a long loop between Axminster and Chard Junction and another by extending the existing loop at Tisbury.  Neither got off the (my) drawing board.

I'll have a look at your 'thoughts' and come back in a few days.

[Oberon]

I've always been amazed the Waterloo-Exeter service can be even partly reliable when there exists such a long single line section as that between Yeovil and Chard Junctions. In a rational world this simply couldn't be, it restricts the route to the barest minimum of trains per day with no space for anything extra than the hourly service. I get the awful feeling that Network Rail are simply not interested in this route to the west country, and are unlikely ever to.

[SandTEngineer]

I've always been amazed the Waterloo-Exeter service can be even partly reliable when there exists such a long single line section as that between Yeovil and Chard Junctions. In a rational world this simply couldn't be, it restricts the route to the barest minimum of trains per day with no space for anything extra than the hourly service. I get the awful feeling that Network Rail are simply not interested in this route to the west country, and are unlikely ever to.

When the line was recontrolled from Basingstoke in 2012 additional mid-section signals were added in both directions at Crewkerne.  This allows the flighting of two trains into the same single line section in the same direction.  This currently allows trains to follow each other at closer headways and is used to good effect when GWR^▸ (Great Western Railway) trains are diverted that way.

One of the other options we looked at in the 1997 study I mentioned in my post above, was extending the double line sections at each end of the single line section to reduce the single line occupation time.  This was looked at for Tisbury to Wilton Junction but the timetable didn't work.  We also looked at doing the same at Pinhoe and that did 'wash its face' but as I stated above, the whole project never left the drawing board......

[SandTEngineer]

Grahame.

Question No.1 - What is the maximum linespeed on the single line?

Question No.2 - What rolling stock is in use/will be in use?

Question No.3 - Is it a mixed traffic line (i.e. Passenger and Freight)?

[PhilWakely]

When the line was recontrolled from Basingstoke in 2012 additional mid-section signals were added in both directions at Crewkerne.  This allows the flighting of two trains into the same single line section in the same direction.  This currently allows trains to follow each other at closer headways and is used to good effect when GWR^▸ (Great Western Railway) trains are diverted that way.

One of the other options we looked at in the 1997 study I mentioned in my post above, was extending the double line sections at each end of the single line section to reduce the single line occupation time.  This was looked at for Tisbury to Wilton Junction but the timetable didn't work.  We also looked at doing the same at Pinhoe and that did 'wash its face' but as I stated above, the whole project never left the drawing board......

I will admit to having a personal interest in having double track reinstated eastwards from Pinhoe, however I am sure that infrastructure challenges such as the M5 motorway bridge and flood prevention in the Cranbrook area would prove too costly (despite potential costs being a drop in the ocean compared to HS2^▸ (The next High Speed line(s)))

[SandTEngineer]

When the line was recontrolled from Basingstoke in 2012 additional mid-section signals were added in both directions at Crewkerne.  This allows the flighting of two trains into the same single line section in the same direction.  This currently allows trains to follow each other at closer headways and is used to good effect when GWR^▸ (Great Western Railway) trains are diverted that way.

One of the other options we looked at in the 1997 study I mentioned in my post above, was extending the double line sections at each end of the single line section to reduce the single line occupation time.  This was looked at for Tisbury to Wilton Junction but the timetable didn't work.  We also looked at doing the same at Pinhoe and that did 'wash its face' but as I stated above, the whole project never left the drawing board......

I will admit to having a personal interest in having double track reinstated eastwards from Pinhoe, however I am sure that infrastructure challenges such as the M5 motorway bridge and flood prevention in the Cranbrook area would prove too costly (despite potential costs being a drop in the ocean compared to HS2^▸ (The next High Speed line(s)))

Yes the M5 bridge is a problem.  I think we looked at a bored tunnel in the motorway embankment for the second track. I don't remember the flood prevention being an issue.   Somewhere I still have a copy of the project file. Over the next few days I'll have a hunt in the garage to see if I can find it.  Sorry if this is getting a bit far away from Grahames original question.....

[grahame]

Grahame.

Question No.1 - What is the maximum linespeed on the single line?

Question No.2 - What rolling stock is in use/will be in use?

Question No.3 - Is it a mixed traffic line (i.e. Passenger and Freight)?

I have two cases which have been suggested - I'll message the details through of I may as I want any silly ideas that have come my way to be quietly stepped past if they don't work.

[Oberon]

When Chris Grayling put forward his recent suggestion that some of the Beechings cuts could be reversed I wish he had realised a great deal of permanent damage to the railways - singling of lines and junctions - were enacted in the 1960s and 70s. Now if he could reverse some of those draconian actions..

[ChrisB]

Would depend on how msny houses might be built as a result. The suggestion was to do more with freeing up land for housebuilding by providing local-to-it transport possibilities than simply opening up old railways

[eightf48544]

Unfortunately any length of of single track in what should be a double track line will impose capacity restraints, depending on its length the linespeed and it's position in relation to other sections of single track.

Whilst a theoretical model could be built for any length of line the optimum would require even length loops with equal length single line sections and a uniform linespeed.   Therefore, unfortunately any actual loops will not necessarily bring very great capacity increases. Witness discussions about Salisbury Exeter.

One scheme that probably reached an optimum increase was Swindon Kemble where double tracK was connected to double track.

I would suggest to get maximum benefit Thingley Jn Trowbridge would need to be doubled throughout and possibly for diversions/regular service Bradford North curve reinstated. IETs^▸ (Intercity Express Train) could then come into their own running Bristol Bath under the wires Bath Thingley under diesel and a  sprint to Padd on the juice. Any form of dynamic loop will be almost as expensive as doubling throughout especially if double track is provided at both ends more  S&C^▸ (Settle and Carlisle ), signals, interlocking  etc


 

[SandTEngineer]

...mmm.  There is a capacity constraint also introduced by having single lead junctions, especially at a place like Thingley Junction where the Down to Up crossover is some distance from the principle single line turnout.  In my experience single lead junctions were a good cost saving device that have now seen their time (and not forgetting they are higher risk than a conventional double junction).

As I mentioned in a  post above, extending the double track at each end of a single line section does have benefits such as allowing a train waiting to access the single line section to stand clear of the main line.  There is nothing new in all this.  Railways of old had the same single line capacity problems.....

At the moment I am looking at a number of options for Grahame, but one early one that does 'wash its face' is the 'flighting' of trains in the same direction by means of additional mid-section signals.

[bradshaw]

The Wessex RUS^▸ (Route Utilisation Strategy) suggests this for Castle Cary-Yeovil Pen Mill as a means of increasing capacity

[RA]

...mmm.  There is a capacity constraint also introduced by having single lead junctions, especially at a place like Thingley Junction where the Down to Up crossover is some distance from the principle single line turnout.

Thingley is certainly a problem now. With the main lines canted for 125mph running, the crossover is some distace away on the nearest section of straight (and therefore level) track. Installing a traditional junction there would be difficult with the track geometry.

The Melksham route fortunately retains space for the reinstatement of a double track formation, with the exception of the bridge over the River Avon at Staverton (Charcroft) which was replaced with a single track span in the 1960s.