The six-foot and the hammer effect

[najaB]

Is there any requirement to increase the width of the 'six foot' as line speeds increase? I'm on a HST at the moment on a fast section of the ECML and the 'hammer blow' when a train passes in the opposite direction is enough to make things on the tables jump. I can't imagine it does the structure of the carriages any good either.

If this is what it's like with a c 250mph closing speed, I can only imagine it would be significantly stronger when the closing speeds are closer to 400mph.

Are there any examples of it doing actual damage e.g. popping a window out?

 

[Tio Terry]

Is there any requirement to increase the width of the 'six foot' as line speeds increase? I'm on a HST at the moment on a fast section of the ECML and the 'hammer blow' when a train passes in the opposite direction is enough to make things on the tables jump. I can't imagine it does the structure of the carriages any good either.

If this is what it's like with a c 250mph closing speed, I can only imagine it would be significantly stronger when the closing speeds are closer to 400mph.

Are there any examples of it doing actual damage e.g. popping a window out?

It is certainly a consideration when planning and designing new HS routes. North Downs Tunnel on HS1 has a 160KPH speed limit on it, not because of the track geometry but because it's a single bore tunnel (unlike French HS lines which use single bore tunnels in the main) because trains passing at higher speeds in the tunnel could cause window damage because of the aerodynamics.

The Channel Tunnel, of course, employs single bore tunnels, not just because of the passing trains issue but because of Fire Safety issues as well.

HS lines are built to the more generous loading gauge employed in mainland Europe. George Hudson has a lot to answer for!

 

[Kentish Paul]

It is certainly a consideration when planning and designing new HS routes. North Downs Tunnel on HS1 has a 160KPH speed limit on it, not because of the track geometry but because it's a single bore tunnel (unlike French HS lines which use single bore tunnels in the main) because trains passing at higher speeds in the tunnel could cause window damage because of the aerodynamics.

The Channel Tunnel, of course, employs single bore tunnels, not just because of the passing trains issue but because of Fire Safety issues as well.

HS lines are built to the more generous loading gauge employed in mainland Europe. George Hudson has a lot to answer for!

I don't think it does. Never been through on a Eurostar but a class 395 does not slow down to pass through after crossing the Medway bridge at 225kmh.

 

[Bald Rick]

Presumably a typo of kph vs mph. It definitely is not a 100mph tunnel!

To answer the OP question, yes it is a consideration. Not only for aerodynamics, but also to enable maintenance to take place on one line with the adjacent line(s) open.

 

[Domh245]

Isn't the "hammer blow" mostly just the doors banging in their frames? Which is more easily solved by pressurised doors such as those on the 395s or IEPs.

 

[Bald Rick]

The Channel Tunnel, of course, employs single bore tunnels, not just because of the passing trains issue but because of Fire Safety issues as well.

Slightly OT, but the Channel Tunnel (and Crossrail, Crossrail 2, and most long bored HS1 and HS2 tunnels, etc etc) are / will be built single bore simply because it is quicker to build. There's much less muck to cart away, much less lining to put in (bigger diameter tunnels need thicker lining). North Downs was done as a single bore as it is perfect tunnelling conditions, relatively short with good access each end, and easy to pump concrete in for the cast in situ lining. (I saw the first casting done myself).

 

[nick.c]

HS lines are built to the more generous loading gauge employed in mainland Europe. George Hudson has a lot to answer for!

In all fairness when the early pioneers designed the first railways in the UK I doubt they ever even dreamed of trains travelling at 125mph.

My first experience of the "hammer effect" on a really fast train was many years ago on a pressure sealed first generation TGV in a tunnel south of Lyon. With a closing speed of at least 300mph to my astonishment there was no discernible "hammer effect". Obviously the track separation (and tunnel cross-section) was far more generous.

 

[najaB]

Isn't the "hammer blow" mostly just the doors banging in their frames? Which is more easily solved by pressurised doors such as those on the 395s or IEPs.

Maybe so, but it's definitely been noticeable on this journey. And you can feel it in the window.

 

[Tio Terry]

Slightly OT, but the Channel Tunnel (and Crossrail, Crossrail 2, and most long bored HS1 and HS2 tunnels, etc etc) are / will be built single bore simply because it is quicker to build. There's much less muck to cart away, much less lining to put in (bigger diameter tunnels need thicker lining). North Downs was done as a single bore as it is perfect tunnelling conditions, relatively short with good access each end, and easy to pump concrete in for the cast in situ lining. (I saw the first casting done myself).

To be bluntly honest, North Downs is single bore because Bechtel saw a cost saving over a twin bore tunnel!

I've done a quick search, and I'm sure there is somebody who knows better than me, but this says there is a speed reduction but not what it is:-

https://en.wikipedia.org/wiki/North_Downs_Tunnel

I know what speed we were told we could use when we were testing systems over the line - and I also know what speeds we actually did!

 

[randyrippley]

Is there any requirement to increase the width of the 'six foot' as line speeds increase? I'm on a HST at the moment on a fast section of the ECML and the 'hammer blow' when a train passes in the opposite direction is enough to make things on the tables jump. I can't imagine it does the structure of the carriages any good either.

If this is what it's like with a c 250mph closing speed, I can only imagine it would be significantly stronger when the closing speeds are closer to 400mph.

Are there any examples of it doing actual damage e.g. popping a window out?

I can't remember details, but there was a news report ~20 years ago of two trains passing at speed in a tunnel. One had a cracked window, which failed due to the pressure pulse, creating debris which caused a cascade of further windows failures down both trains. From what I remember the windows didn't pop - the panes just shattered. Sorry can't remember details - far too long ago

 

[47271]

Is there any requirement to increase the width of the 'six foot' as line speeds increase? I'm on a HST at the moment on a fast section of the ECML and the 'hammer blow' when a train passes in the opposite direction is enough to make things on the tables jump. I can't imagine it does the structure of the carriages any good either.

If this is what it's like with a c 250mph closing speed, I can only imagine it would be significantly stronger when the closing speeds are closer to 400mph.

Are there any examples of it doing actual damage e.g. popping a window out?

Was this First Class on a Grand Central HST by any chance? I swear that every single teacup jumps an inch off the table every time they pass something between Thirsk and York. It doesn't seem quite so extreme on VTEC trains, but I think that's only because they don't use saucers!

It's great for waking up anyone trying to have a snooze en route.

 

[coppercapped]

Maybe so, but it's definitely been noticeable on this journey. And you can feel it in the window.

The 'thump' on the side of HST coaches is because the sides are sheet steel spot welded to a lightweight frame and this combination is slightly flexible. The top is stiffened by the cantrail structure and the bottom to the floor but mid-height it can be pressed in slightly. The pressure pulse at the front of the train coming in the opposite direction is enough to give the 'thump' - a bit like a drum skin.

It is not dangerous, no windows - to my knowledge have ever been 'popped out' - they are fixed quite securely - and the trains have been running for some 40 years with no signs of bodyside fatigue.

Extruded aluminium planks with internal stiffening - now often used in coach construction - don't show the same effect.

 

[najaB]

It is not dangerous, no windows - to my knowledge have ever been 'popped out' - they are fixed quite securely - and the trains have been running for some 40 years with no signs of bodyside fatigue.

Oh, I didn't think it was dangerous just wondered if it was something that had to be considered when designing high speed rail lines.
--- old post above --- --- new post below ---

Was this First Class on a Grand Central HST by any chance? I swear that every single teacup jumps an inch off the table every time they pass something between Thirsk and York.

It was actually first class on a VTEC HST.

 

[47271]

I'm grateful to learn that this effect is due to the construction of mk3 coaches.

I'd always assumed that the tea cups rattle because of air pressure through the end droplights not being fully shut, although now that I think about it this would be unlikely with the internal vestibule doors closed.

Off topic, it's still much preferable to what the underfloor buzz does to the cups on GC's 180s.

 

[Crossover]

It affects the single seats in first class most as the tables are fastened to the wall of the train and are not secured to the floor by a leg. I have literally seen things jump before now when passing other trains!

 

[edwin_m]

I'm told by someone who has researched this subject that 140mph is about the limit for the traditional six-foot track spacing. The risk is ballast being dislodged between passing trains, potentially causing damage and injuries. So for anyone who favours increasing speeds on the WCML instead of building HS2, it would involve re-spacing the tracks and therefore re-building at least half of the OLE, signals, stations, structures etc (although perhaps slab track would be an alternative?).

 

[BestWestern]

Isn't the "hammer blow" mostly just the doors banging in their frames? Which is more easily solved by pressurised doors such as those on the 395s or IEPs.

Only on Class 150s etc, and those doors rattle with anything more than a light breeze!

I was once told by a manager, truthfully or otherwise, that it was found in a certain tunnel (I don't know where) that two HSTs passing at full chat caused the lavatories to empty themselves all over the floor, with the speed limit having to be lowered accordingly. I have no idea if that is truth or managerial myth!

 

[AM9]

Only on Class 150s etc, and those doors rattle with anything more than a light breeze!

I was once told by a manager, truthfully or otherwise, that it was found in a certain tunnel (I don't know where) that two HSTs passing at full chat caused the lavatories to empty themselves all over the floor, with the speed limit having to be lowered accordingly. I have no idea if that is truth or managerial myth!

It's also true for Class 319s passing each other on the MML or an HST/Meridian at 80mph+.

 

[paul1609]

It is certainly a consideration when planning and designing new HS routes. North Downs Tunnel on HS1 has a 160KPH speed limit on it, not because of the track geometry but because it's a single bore tunnel (unlike French HS lines which use single bore tunnels in the main) because trains passing at higher speeds in the tunnel could cause window damage because of the aerodynamics.

The Channel Tunnel, of course, employs single bore tunnels, not just because of the passing trains issue but because of Fire Safety issues as well.

HS lines are built to the more generous loading gauge employed in mainland Europe. George Hudson has a lot to answer for!

The limit through North Downs tunnel is 300 kph.

 

[Peter Mugridge]

I was once told by a manager, truthfully or otherwise, that it was found in a certain tunnel (I don't know where) that two HSTs passing at full chat caused the lavatories to empty themselves all over the floor, with the speed limit having to be lowered accordingly. I have no idea if that is truth or managerial myth!

I have never heard of this with HSTs, but when the Mk4 stock was introduced, it was found while they were still on test that this happened in the Grantham tunnels and there was a speed restriction on them until a technical solution* could be developed.

This "£waterspout effect" happened with single rakes; it did not need another rake passing in the opposite direction.


*Probably stronger hinges on the flap valves or whatever they are called in the retention tanks?

 

[158747]

I was once told by a manager, truthfully or otherwise, that it was found in a certain tunnel (I don't know where) that two HSTs passing at full chat caused the lavatories to empty themselves all over the floor, with the speed limit having to be lowered accordingly. I have no idea if that is truth or managerial myth!

I have heard of this too. It refers to Alderton Tunnel on the Badminton line between Swindon and Bristol Parkway. The line speed each side of the tunnel is 125mph, the speed through the tunnel being 110mph. As the tunnel has no ventilation shafts the excess air pressure when two HSTs passed was then forced up into the toilets, I feel sorry for whoever it was who was the first to experience this problem!

 

[nick.c]

Is there any requirement to increase the width of the 'six foot' as line speeds increase?

The distance between track centres on French LGV's is 4.5m for a design speed of 350 km/h. For HS2, with its 400 km/h aspiration, track separation will be up to 5.0m

So, with good aerodynamics, strong construction and generous track spacing, on HS2 the "hammer effect" should be a thing of the past. I hope so anyway. In the peaks an HS2 service will be passing a train on the opposite track every 90 seconds.

 

[Peter Mugridge]

I have heard of this too. It refers to Alderton Tunnel on the Badminton line between Swindon and Bristol Parkway. The line speed each side of the tunnel is 125mph, the speed through the tunnel being 110mph. As the tunnel has no ventilation shafts the excess air pressure when two HSTs passed was then forced up into the toilets, I feel sorry for whoever it was who was the first to experience this problem!

Interesting.... this must have been meany years before the Mk4 problem, so I suspect it may not be the last time it occurs either.

Memo to self: Avoid going to the toilet on brand new types of rolling stock when approaching a tunnel...

 

[QueensCurve]

Isn't the "hammer blow" mostly just the doors banging in their frames? Which is more easily solved by pressurised doors such as those on the 395s or IEPs.

Or perhaps plug and swing doors could be used.

 

[70014IronDuke]

Interesting thread, but surely this is not referred to as 'hammer blow' ?

Pressure wave, or pressure blow, or some such would be more apt.

As I understand it, classical 'hammer blow' is the vertical forces exerted on track by unbalanced rotating mass, most especially those created by the motion on steam locomotives.

Muddling technical terms, I for one like it not.

disgusted of Wellesley House, etc etc

 

[najaB]

Interesting thread, but surely this is not referred to as 'hammer blow' ?

Pressure wave, or pressure blow, or some such would be more apt.

Quite possibly. I was describing the effect rather than using a technical term (as it definitely felt like someone hit the side of the train with a large hammer/mallet). I suppose it would have a boring name like 'dynamic over-pressure pulse' or similar.

 

[Crossover]

I have heard of this too. It refers to Alderton Tunnel on the Badminton line between Swindon and Bristol Parkway. The line speed each side of the tunnel is 125mph, the speed through the tunnel being 110mph. As the tunnel has no ventilation shafts the excess air pressure when two HSTs passed was then forced up into the toilets, I feel sorry for whoever it was who was the first to experience this problem!

Not for the same reason but I'm sure there is a limit on HSTs in Severn Tunnel - one is not allowed to pass the other or something? Not sure if it is the same reason or something other

 

[158747]

Not for the same reason but I'm sure there is a limit on HSTs in Severn Tunnel - one is not allowed to pass the other or something? Not sure if it is the same reason or something other

Freight trains carrying hazardous goods are not allowed to pass other trains in the Severn Tunnel, If one is going through the tunnel, train travelling in opposite dirction will be held outside tunnel until freight train has passed through. Or vice versa if freight is about to enter tunnel when there is train already in tunnel travelling in opposite direction.

 

[matchmaker]

What is being described is not "hammer blow".

 

[61653 HTAFC]

With regard to the toilet issue, I'd assume that the causes differ between mk3 and mk4 stock. Mk3s at the time dumped onto the track, so "pressure waves" could find their way into the system that way. Mk4s were built with waste tanks which one would hope is a closed system, so the cause must be something else.