Why does everyone love pairing by use, for 4-track railways

[zwk500]

I don't think the stations are as problematic as you might think.

If they're not that problematic why do the current slows drop down to 75/90mph through most of them?

My thinking was slows on the middle to ease the termination issues, but the point is equally valid for fasts in the middle. Possibly even more so.

This will make junctions around Wembley fun...

Either way, one of the existing fast lines remains a fast line, and you only have to upgrade one of the slow lines to achieve your goal, and that can all be done whilst it's still the slow line.

You have to upgrade it quite a bit though, and that's even harder if you need to maintain the ability to stop trains regularly on the platform as you do so.

At a glance, all the station approaches actually look pretty straight it's things like the approaches to the Watford tunnels where the alignment appears problematic. Arguably there a better solution there might be to bore a new tunnel west of the existing ones, and leave the fasts using the current one, rather than having to acquire the land to re-profile the current slow line approaches. Dunno if that's worse or not. IIRC, many years ago there was talk of a 5th line or even 6 tracking out to Hemel or Kings Langley to ease freight congestion, which would have been just as expensive, so perhaps not entirely fanciful.

I don't think it'd be impossible by any means - just more effort than simply rejigging a few sets of points at Hanslope. I also don't think it'd be worth the money even if it were simpler than I think.

 

[mr_jrt]

If they're not that problematic why do the current slows drop down to 75/90mph through most of them?

I would imagine:

1. Passengers are more likely to be standing on those platforms if they don't have fencing
2. Whatever the technical term for the tamping/distance from the coping stones. The fast lines can be optimised for line speed through the platforms, with the lines tamped slightly further away or with more cant, whereas the slow lines need to minimise it as trains regularly stop at them?
3. Quite likely also localised features like S&C which will introduce their own limits that aren't actually big limits in a fundamental sense, like having an important building in the way would be. They're all easily resolvable as long as the land is available.
   

 

[The Planner]

I would imagine:

1. Passengers are more likely to be standing on those platforms if they don't have fencing
2. Whatever the technical term for the tamping/distance from the coping stones. The fast lines can be optimised for line speed through the platforms, with the lines tamped slightly further away or with more cant, whereas the slow lines need to minimise it as trains regularly stop at them?
3. Quite likely also localised features like S&C which will introduce their own limits that aren't actually big limits in a fundamental sense, like having an important building in the way would be. They're all easily resolvable as long as the land is available.

They got cost cut as part of WCRM. They would have gone up to 100 or more.

 

[zwk500]

They're all easily resolvable as long as the land is available.

This is indeed a key question.

They got cost cut as part of WCRM. They would have gone up to 100 or more.

On the same alignment?

 

[The Planner]

This is indeed a key question.

On the same alignment?

Yes.

 

[MarkyT]

One reason why paired by use has been quite popular is that it allows 4-platform stations to take up a limited footprint as the fast lines continue through and the slow lines can make the compromise of a tighter curve between the island and the outer platform.

Not everywhere. On GWR, the mains do the platform wiggle at Twyford, presumably the reliefs being the original straight route there. Known as the Twyford twitch, it's a useful wakeup reminder for express commuters alighting at Reading.

== Doublepost prevention - post automatically merged: 16 Oct 2024 ==

Off the top of my head, Windsor lines approaching Waterloo are PBD. All others are either 2/3 track approaches, or PBU (either all the way to the station or with a brief bit of fully reversible working for the final throat).

As far as Carlisle Lane Jn, just before West Crossings, the Windsors are PBD on final approach. That makes it easier for passengers waiting for the next train to Clapham Jn at Vauxhall. Added complications are the 3-track section through Nine Elms flyover and Queenstown Rd station, and the Windsor reversible; full bidi on the northernmost line, often previously used in both directions by Eurostars between Waterloo International, the continent, and the old North Pole depot. Having two down lines next to the up main fast is useful as the nearer one can be used in morning peak for emply stock release from the terminal back to Clapham Junction or Wimbledon. That it's to the north of the up fast means there are some combinations where parallel junctions can allow an empty to slip out to the Down Windsor Fast from a higher-numbered platform at the same time as an up express is arriving at a lower-numbered platform. I used to think PBD PBU would be good along here but track layout constraints, service patterns etc don't make it so easy.

Edit: last sentence corrected.

 

[zwk500]

As far as Carlisle Lane Jn, just before West Crossings, the Windsors are PBD on final approach. That makes it easier for passengers waiting for the next train to Clapham Jn at Vauxhall. Added complications are the 3-track section through Nine Elms flyover and Queenstown Rd station, and the Windsor reversible; full bidi on the northernmost line, often previously used in both directions by Eurostars between Waterloo International, the continent, and the old North Pole depot. Having two down lines next to the up main fast is useful as the nearer one can be used in morning peak for emply stock release from the terminal back to Clapham Junction or Wimbledon. That it's to the north of the up fast means there are some combinations where parallel junctions can allow an empty to slip out to the Down Windsor Fast from a higher-numbered platform at the same time as an up express is arriving at a lower-numbered platform. I used to think PBD would be good along here but track layout constraints, service patterns etc don't make it so easy.

For the windsors, my ideal state would be to remove the old Eurostar flyover, change the track layout to paired by use with fasts on the south and slows on the north, rebuild Queenstown road to be an island in between the slows, and remodel Barnes Jn appropriately, and eliminate conflicting moves at Barnes, with all trains from Richmond running fast between Barnes and Clapham Junction, and all trains on the slow lines running via the Hounslow Loop.
This would allow the former Int'l platforms at Waterloo to be the 'Hounslow' platforms, and also mean conflicting trains crossing to/from the other connections on the north side between Clapham Jn and Queenstown Road only have to cross 1 slow line, not both Up lines.

I accept it's a fantasy though.

 

[Falcon1200]

I'm guessing the reason that arrangement with slow lines in the middle works there is that the fast Met lines aren't actually that fast - it's only about 5 miles from Finchley Road to Wembley Park and the Met trains don't go at a massive speed between them, so there's not really an issue with the fast lines splaying out to accommodate the centre platforms.

Not an issue nowadays, but in the past an argument against Fast Lines outside Slows would be serving goods yards and sidings!

 

[jscz]

The problem with this is that any two-track sections (e.g. tunnels/viaducts never quadrupled) would involve the fast lines merging into the slow lines, causing speed bottlenecks - even the fastest junctions would involve fast trains having to slow down. By having the fast lines in the middle, the slow tracks (which have trains going at lower speeds anyway) are the ones merging in.

== Doublepost prevention - post automatically merged: 27 Oct 2024 ==

In the absence of flyovers, which pairing is better, use or direction, and if paired by use, which side do you put the slow lines on?

I think if you have multiple diverging lines with no option for grade separation (i.e. flyovers/unders), then paired by use is probably easier as it makes for smoother junctions. However, the practicality is that whatever you do in this situation, the capacity will be lower using level junctions. That's why most of the time the southern ECML sucks it up and just builds a flyover/under (aap, wgc, svg)

 

[The Planner]

The problem with this is that any two-track sections (e.g. tunnels/viaducts never quadrupled) would involve the fast lines merging into the slow lines, causing speed bottlenecks - even the fastest junctions would involve fast trains having to slow down. By having the fast lines in the middle, the slow tracks (which have trains going at lower speeds anyway) are the ones merging in.

That doesn't need to be the case. You are assuming that the fast lines are the ones joining on slow S&C, you can still have the fast lines as the main route and the middle slow lines being the ones that join.

 

[furnessvale]

I think if you have multiple diverging lines with no option for grade separation (i.e. flyovers/unders), then paired by use is probably easier as it makes for smoother junctions. However, the practicality is that whatever you do in this situation, the capacity will be lower using level junctions. That's why most of the time the southern ECML sucks it up and just builds a flyover/under (aap, wgc, svg)

Those 60 or so years of major european railways investing that little bit extra every year in building flyovers certainly shows in timekeeping when compared to GB.