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Offline Geezer

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« on: 11/03/2010 05:56:41 »
Wowsers! At last, a really exciting topic. Yeah!!

Before the days of continuous welded rail, railway tracks were assembled with relatively short sections of rail that were connected by means of rail joiners, also referred to as "fishplates" (to find out more on fishplates you'll have to wait for even more exciting future topics).

Ahem! Anyway, for reasons unknown, in the UK the joiners were always aligned so that both rails had their joiners interposed between one set of sleepers (ties) whereas, in the US, the joiners were staggered so that the maximum distance was attained between joiners on opposite rails.

This could be useful information. If you ever find yourself kidnapped and blindfolded on a train, you might be able to determine if you are in the UK or the US from the sound as the train crosses the rail joints.

However, why would the two systems be so different? Which one was better?


 

Offline graham.d

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« Reply #1 on: 11/03/2010 08:41:12 »
Fascinating. I don't know the answer; however it interesting (slightly) to think that the rhythm would be twice as fast on the US trains, but as the wheels are on bogeys of about the same size in the UK as the US, the pair of klunks from the bogey going over the joins would enable approximate mental calibration of the speed.

 

Offline LeeE

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« Reply #2 on: 11/03/2010 16:46:51 »
Hmm... dunno the definitive answer to this, but wear is exacerbated at the rail joints.  This is because the two rails must be able to move relative to each other, to allow for expansion and contraction, but this also means that the ends of the rails also dip very slightly as the wheels roll over them (and it is this 'dipping' of the rail ends that produces the jolt (combined with 'pumping' see below).  I was quite alarmed as a youngster when I first observed just how much the rail ends at the joints actually moved when a train passed over them - up to ~1/4 of an inch.

With staggered joints, the combination of 'dipping' and increased wear would tend to induce a rolling action as the vehicle passed over them, but reduce the jolt.  With the joints aligned you wouldn't get the rolling action but the jolt would be worse.

Historically, railways in the U.S. weren't laid to the same standard as the U.K. and generally offered a poorer ride (the heavier loads didn't help track quality either), so perhaps the reason for staggering the joints was an attempt to improve comfort by reducing the jolt severity at the expense of slightly increasing roll.  I would imagine though, that aligned joints might be a bit safer, because of the reduced roll.

There's also another factor that exacerbates the problem too: that of 'pumping'.  As the track is pressed down and then released, as each train passes, it can create cavities beneath the sleepers, throwing the track out of alignment.  Between the joints this may result in an incorrect camber i.e. track that should be level ends up dipping down on one side, but at the rail joints it tends to be worse, but more even, and just adds to the amount of dip over the joints.  The combination of pumping and joint dip can lead to the rail ends moving up and down by up to 3/4 of an inch as a train passes over, something I also observed as a youngster.
 

Offline Geezer

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« Reply #3 on: 11/03/2010 19:06:00 »
Many good points! (ha ha) - (Dave may not get that one.)

I wondered if the stagger helps to maintain alignment on curves - one rail acts as a sort of splint for the opposite joiner.

In a passenger train in the UK you typically hear four beats with a slightly longer space between the second and third beats. This is followed by a longish pause then it repeats. Two of the four beats come from the bogie in your coach and the second pair come from the adjacent bogie in the next coach.

As I recall, it's a bit difficult to make out a distinct rhythm in a US passenger train. I sounds more like a continuous stream of beats. I suppose if we want to know, we'll have to get data on the rail lengths, bogie (truck) centres pitch, bogie axle spacing and coupler spacing (no buffers of course).

BTW1, did you know that the old fashioned cast iron chairs that support bullhead rail in the UK actually tilt the rails over slightly so that the top surface of the rail corresponds to the conical profile of the wheel treads?

BTW2, did you know that continuous welded rails are actually stretched? They are under a quite a bit of tension. Only the ballast around the sleepers (ties) keeps them in the desired position.
« Last Edit: 11/03/2010 20:56:36 by Geezer »
 

Offline LeeE

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« Reply #4 on: 12/03/2010 00:16:32 »
BTW1, did you know that the old fashioned cast iron chairs that support bullhead rail in the UK actually tilt the rails over slightly so that the top surface of the rail corresponds to the conical profile of the wheel treads?

Yup, except I think that bullhead rail stopped being used for running lines back in the 1940/50s and only 'T' section rail is used now (you might still find it on sidings and preserved railways though).  Conical wheel profiles also went 'out' in the 70/80's, and were replaced with 'worn' profiles for all high speed traffic.
 

Offline Geezer

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« Reply #5 on: 12/03/2010 03:56:27 »
BTW1, did you know that the old fashioned cast iron chairs that support bullhead rail in the UK actually tilt the rails over slightly so that the top surface of the rail corresponds to the conical profile of the wheel treads?

Yup, except I think that bullhead rail stopped being used for running lines back in the 1940/50s and only 'T' section rail is used now (you might still find it on sidings and preserved railways though).  Conical wheel profiles also went 'out' in the 70/80's, and were replaced with 'worn' profiles for all high speed traffic.

Right. I don't think there is any bullhead rail in use on main lines. I think BR experienced some problems when they started to switch over to flatbottom rail. There were lots of experiments with different types of sleepers and clips.

Edit: Just a thought. The mass of all those cast iron chairs must have produced a non-trivial amount of inertia. I wonder if the calculations during the switchover to flatbottom rail took that into account?
« Last Edit: 12/03/2010 08:17:15 by Geezer »
 

Offline graham.d

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« Reply #6 on: 12/03/2010 10:45:36 »
I didn't know about the tilt of the rails but did know about prestretching to allow "relaxation" of the stretch rather than buckling in hot weather. Before they used this they did have long track with long tapered overlaps at the joins. You never see these around today though and I only saw them at a railway museum and never on a real track.

I expect you guys know, but many don't, that the taper on a train wheel (the part nearer the flange has a bigger diameter than the part on the wheel facing outward) so that if the train "wanders" to one side, the change in the relative diameters of the wheels on each side (joined by a rigid axle) steers the train back. Most people think the train is kept on the track by the flange but this is not so except when going at non-optimum speeds around bends.

Another interesting fact (at least to "sad" physicists) is that, where used, the overhead high voltage line is typically suspended from another line by a number of vertical connecting lines - the idea to keep the pickup line as level as possible - and that the shape of the top supporting line (between its main supports) forms the approximate shape of a hyperbolic cosine.
 

Offline Variola

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« Reply #7 on: 12/03/2010 12:53:17 »
In the UK we have track circuits, its an electric circuit and the voltage drops when the train hits the fishplate. There is a relay on the circuit which then drops and signals back to where the train is, so the railways can be divided up into sections, and you can see where which train is at all times.So it is important the fishplates are parallel.

In the USA they use axle counter circuits, the the axles of the train are counted, as they pass over the fishplates by means of magnetic field affecting the voltage, so enabling the train position can be tracked that way, and usefully with the long American trains, whether it is all still complete. That system does not reply on the fishplates  being parallel.

I should hastily add here that, that is from my O/H who is a train geek, I asked him when he phones just now! As for why the US fishplates are not parallel, he has no idea as it is easier for track repair to make them parallel. 
 

Offline LeeE

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« Reply #8 on: 12/03/2010 18:15:16 »
I expect you guys know, but many don't, that the taper on a train wheel (the part nearer the flange has a bigger diameter than the part on the wheel facing outward) so that if the train "wanders" to one side, the change in the relative diameters of the wheels on each side (joined by a rigid axle) steers the train back. Most people think the train is kept on the track by the flange but this is not so except when going at non-optimum speeds around bends.

Up to the 1960's, U.K. railway wheels normally had a 1:20 flat taper across the tyre width, whereas most European wheels had a 1:40 flat taper across the inner ~2/3rds of the tyre, with the outer third being increased to 1:10 to increase the self-centering action when the wheel deviated too much.  During the 1960's though, the pseudo 'worn' wheel profile was developed, which had a much flatter taper but with a deliberate slight hollow across the profile, hence being termed a 'worn' profile.  These pseudo 'worn' profiles were developed to reduce hunting with new wheelsets at high speeds and which ultimately allowed the development of the high speed trains we have today, which wouldn't have been stable with the old conical tapered wheels.

I couldn't find any simple references to these pseudo 'worn' profiles, because I guess they are a bit obscure, but the wikepedia article about the British Rail Research Division briefly mentions them:

http://en.wikipedia.org/wiki/British_Rail_Research_Division

"One of the first major projects was the development of profiled/pre-worn wheels which helped counter the tendency of new wheels to hunt. This led to research into vehicle suspensions, and the creation of the four-wheel High Speed Freight Vehicle (HSFV1) which proved stable at up to 140mph when tested on the roller rig."

You can also find a few technical documents via a google search on 'rail wheel profile p8'

It is certainly true, as graham.d says, that it is not the flange that keeps the wheels on the tracks, but the profile of the wheel: the flange is only there as a 'last resort' safety feature.  Were the flanges to be in frequent contact with the rails, both the flanges and the sides of the railhead would soon be worn away.

Also, most of the 'squeal' you hear when a train is negotiating a tight curve doesn't come from flange contact, either with the side of the railhead or with the check rails, but from the fact that the axles don't have a differential between the two wheels, with the result that one or both of the tyres have to slip on the railhead, very slightly, to cover the differing distances along the inner and outer rails as the axles goes around the curve.
 

Offline Variola

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« Reply #9 on: 12/03/2010 18:39:42 »
Lee if you were a woman my O/H would think you were fantastic!!!  :o :)
Graham D would be a close second too!!

I will show him this thread when he is over tomorrow, if you spot a reply on here from me full of train wisdom, then it's him...!


 

Offline Geezer

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« Reply #10 on: 12/03/2010 21:48:12 »
"Ooooooo! I luff it when you talk trainy to me."
 

Offline Bored chemist

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« Reply #11 on: 13/03/2010 00:39:20 »
Sorry to be a git, but I thought that rails were welded these days.
 

Offline Geezer

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« Reply #12 on: 13/03/2010 01:19:21 »
Sorry to be a git, but I thought that rails were welded these days.

Yes, they are on the main lines, but I believe there is still a fair amount of track in use that is not continuously welded.

I think the modern signalling arrangements use hall effect sensors, or at any rate, they did at one time.

There are vast chunks of the US rail system that have no signalling or train detection at all (only used for freight of course.) Everything is handled by radio between the operators and the dispatchers. If the operator forgets to return a switch (point) to the normal position, bad things can happen, and they do from time to time.

« Last Edit: 13/03/2010 01:25:08 by Geezer »
 

Offline Variola

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« Reply #13 on: 13/03/2010 08:03:02 »
"Ooooooo! I luff it when you talk trainy to me."

That is a phrase that never leaves my mouth....!!!!
 

Offline JimBob

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« Reply #14 on: 14/03/2010 02:45:18 »
I thought "rail" joints were the thin ones that didn't have  much MJ in them.
 

Offline Dudley

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« Reply #15 on: 22/03/2010 09:44:41 »
FB rail had been tried out in the UK in the 1930s, I believe, but in those days there was plenty of labour to maintain bullhead track in good condition - remember rails were held in the chairs by wooden wedges, which contracted in dry weather and fell out, and soon had to be replaced.  Hence the gang which walked the line every day, replacing fallen wedges.  Economics/finances forced the adoption of FB rail after WW2.  All renewals, well, pretty well all, used FB rail with concrete sleepers.  These were found better than wood or steel, heavier, stronger, longer lasting (after they had determined the best proportions of mix - cracking not desired) and used elastic pads between rail and concrete.
Note rails in the UK were a standard length of 60 ft - in the US 39 ft.  Why?  Dunno.  But again in the 1930s the LMS (I think) wanted to go to longer rails - 120' - and asked if these could be rolled.  Reply - "We roll them in 240' lengths and cut them to 60' because that is what you ask for."
London Transport was the standout using bullhead as that was easier to deal with in the confined spacing of the tube, but I understand that it is now changing to FB track for renewals.
As an alternative to the long overlap rail joint, a long fish plate was developed which provided half the rail head as well as the normal material.  This would have been more rigid, and removed the 'clickety-click' but I believe the cost of machining half the railhead away may have been one of the causes it was not adopted on a large scale.
Re coning, BR and LTE experienced severe hunting in certain trains at certain speeds, and I believe it may have been noticed that trains just about due for wheel lathing had less hunting than trains with new wheels.  BR did a lot of research, and eventually developed the worn wheel profile -in effect there is negligible coning over much of the tread, but it increases to the flange, with a large radius for the junction between tread and flange.  This means that under normal conditions the wavelength of sinusoidal curving of the wheelset is very large, due to the very small coning, but in curves the differential in diameters is greatly increased, so curving problems are reduced.  Add dampers and very good riding is found.  Hunting occurs in a small speed range determined by gauge, wheel diameter, wheel shape, axle load, wheel mass and inertia, curve radius and possibly wideness to gauge of rails.  At greater speeds hunting does not occur, and while some railways reduced the coning from 1:20 to 1:40 or even 1:80 others, I believe, tried coning as steep as 1:14 or even 1:7 so that the hunting would only occur at a mid range speed and normal speed would be above the resonant speed.
 

Offline LeeE

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« Reply #16 on: 22/03/2010 19:26:26 »
Although bullhead rail was initially held in place by wooden wedges, these were largely replaced by extruded steel 'S' shaped sprung wedges on track that saw regular running, before being superseded by flat-bottom rail.  You can see one of these 'S' wedges in the chair at the left of the pic linked to below (I won't inline it because it's a pretty big image)

http://commons.wikimedia.org/wiki/File:Fishplate_joining_two_sections_of_bullhead_rail_at_Cardiff_Bay_railway_station_01.jpg

These steel wedges were obviously not effected by humdity, but just like the wooden wedges, were still subject to (and didn't prevent) rail creep (where the passage of trains gradually shifts the rails through the chairs. This eventually leads the the expansion gaps closing up, at which point the track would need to be lifted and pulled back to restore the expansion gaps.  Failure to do this was the cause of the Tattenhall Junction rail crash on the 2nd of July 1971 see:http://www.railwaysarchive.co.uk/eventsummary.php?eventID=131 where you can download the full DoE accident report)
 

Offline Geezer

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« Reply #17 on: 22/03/2010 20:23:10 »
I think Lee is quite right. Also, concrete sleepers were used from quite early in the introduction of FB rail, but there was a still a huge amount of FB rail on wooden sleepers.
 

Offline LeeE

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« Reply #18 on: 23/03/2010 17:24:00 »
Way back in the very early days of railways, before the block working system was devised and when breakdowns were relatively common, one railway company in the U.K. introduced a rule whereby the train crew of a breakdown were to light fires between the rails behind the train to protect it from following trains.  Needless to say that this did nothing at all for the integrity of the track and the practice/rule didn't last long.

(Before block working was introduced trains used to be dispatched using the 'Minimum Interval' system, which just amounted to holding a following train until a certain amount of time had elapsed since the preceding train had passed.  With no way of synchronising clocks between stations and signal boxes, and 'local' time differing between the various stations and signal boxes by amounts much greater than the Minimum Interval, this was rather less than effective and the only real safeguard was for the footplate crew of following trains to keep a good lookout (although the relatively low speeds of the trains helped somewhat).  However, if a train broke down in a tunnel, where the smoke would obscure the view of the footplate crew on a following train, or on a curve in a cutting, a rear collision with the breakdown was highly likely.  Scary days indeed, reflected by the number of such accidents)
 

Offline techmind

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« Reply #19 on: 29/03/2010 23:54:50 »
Thanks for an interesting thread, folks.

Despite my dad and brother building model steam engines, and having a 5-inch gauge track in the garden, I'd never before given thought to wheel profiles and any potential issues of 'hunting' at speed.

Just one thing to add: when I was in Japan and travelled on their high speed trains, my recollection (from 8-10 years ago) was that they laid their track on continuous concrete. No sleepers and ballast. This definitely applied to open-air stations, and I'm fairly sure to the whole track. I imagine this reduces maintainance as the track can't 'wander' over time - certainly not movement relative to the platform edge (there wasn't a big gap like we have in the UK).

I wonder what the other implications or downsides are to mounting the track on continuous concrete...?


I should also say that in Japan they ran exactly the same timetable 7-days a week. None of this special Saturday and Sunday timetabling, and half the network closed for engineering works every other weekend that we put up with here.
 

Offline Geezer

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« Reply #20 on: 30/03/2010 04:06:32 »
Crikey! (as Bunter would have said) Techmind has brought up model railways! I suppose we might be diverging somewhat from the original topic. If there are strong objections, I'll move this offshoot to "Conspiracy Theories" or something.

Ahem! This is actually vaguely scientific. (Well at least it's about standards.) Now might be a good time to take a whizz or get a cup of tea or something. This is a bit verbose.

As some you are probably aware, there are many different scales of model railways. In the UK, one of the most popular is OO which was made very popular by Hornby with the Hornby Dublo range. Consequently, that scale became a defacto standard in the UK. The scale is actually about 1:76 and the track gauge is 16.5 mm. Meanwhile, just about everwhere else in the world, adopted a scale of 1:87 with a track gauge of 16.5 mm and called it HO (as in "half O") and that became the defacto standard outside the UK.

A lot of UK modellers were not happy with the fact that OO models are running on track that really looks seven inches (as those of you with slide rules are already aware) too narrow compared to the real thing. It really does look wrong to a lot of people. So, many of those modellers changed gauge to something correct to scale. (It's a lot easier to change gauge than it is to scale everything else down.)

Anyway, things went along reasonably well for about fifty years or so. After all, very few pieces of equipment operate on both sides of the English Channel, so who cares? Uh oh! Then the Channel Tunnel opened.

Now there is a lot of stuff running on both sides of the Channel, and it seems likely there will only be more over time.

This does not seem like a good situation for the model manufacturers, or their customers. The manufacturers are hardly likely to want to build two models of everything in two different scales, but the customers (on either side of the Channel) will want to be able to buy models from any supplier. But they won't mix different scales side by side, because they look pretty stupid! (Trust me - I have some.)

Admittedly, many members of the community could really care less about this, because many of us are modeling things that existed in reality a long time ago. So, for many of us, it's not really a problem. However, I think it will become a bigger problem over time as newer members want to model newer equipment.

Soooooo, I brought the full force of my legendary problem solving skills to bear on this one and came up with, even though I say it myself, the following rather brilliant solution.

As the ROW is unlikely to convert to OO, it seems that, ultimately there will be a showdown, and OO will become the Betamax of railway modelling. However, that does not mean there has to be a major discontinuity. The OO manufacturers could gradually migrate towards HO. The trick would be that as new models of new real world equipment were introduced, they would be gradually scaled down until, in some fairly large number of years, everything was being built to HO scale. Pretty cool, eh?

Now everybody is happy. Everything that would be seen together in real life is going to be at, or very close to, the same scale. I immediately penned a letter to the model railway press knowing that everyone would immediately seize this idea. A Nobel prize might even be forthcoming.

Er, well, not exactly! I eventually got my letter printed. There was a comment from the editor, but he clearly didn't exactly grok the idea. Even worse, nobody even took the trouble to write in and point out that I was obviously out of my gourd!

Sigh!
« Last Edit: 30/03/2010 06:54:07 by Geezer »
 

Offline LeeE

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« Reply #21 on: 30/03/2010 08:46:58 »
Just one thing to add: when I was in Japan and travelled on their high speed trains, my recollection (from 8-10 years ago) was that they laid their track on continuous concrete. No sleepers and ballast. This definitely applied to open-air stations, and I'm fairly sure to the whole track. I imagine this reduces maintainance as the track can't 'wander' over time - certainly not movement relative to the platform edge (there wasn't a big gap like we have in the UK).

I wonder what the other implications or downsides are to mounting the track on continuous concrete...?

An interesting point.  Yes, most of the pictures I've seen of the Shinkansen show them running on concrete bedded track.

However, Wikipedia says:

Quote
A combination of ballasted and slab track are used, with slab track exclusively employed on concrete bed sections such as viaducts and tunnels. Slab track is significantly more cost-effective in tunnel sections, since the lower track height reduces the cross-sectional area of the tunnel, thereby reducing construction costs by up to 30%.

From: http://en.wikipedia.org/wiki/Shinkansen#Track

indicating that ballasted track is used as well.  The fact that that some sections of the Shinkansen tracks are kept clear of snow by sprinkling hot water over them made me think about the effects of water run-off (either from the heated snow clearing water, or just from the rain) upon the stability of concrete bedded track laid on top of ordinary soil; the water run-off would be likely to undermine a solid and rigid concrete bed but it wouldn't be apparent until the cavities became very large and the track shifted catastrophically.  With ballasted track though, as cavities form due to run-off, the individual lumps of ballast can shift to fill them without the risk of a large cavity forming.  On the elevated sections of track, or where the track is laid on top of more substantial foundations, the water run-off can be diverted to drains.

Just guessing here, of course.

One thing that I am pretty sure of though, is that the concrete bedded track will be much more 'solid' and 'firmer' than the ballasted track, and that the Shinkansens will need some clever suspension to be able to handle the difference between the two types.
 

Offline Geezer

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« Reply #22 on: 25/04/2010 23:59:26 »
Apparently maglev high speed trains are now being seriously considered in the US.

Maglevs seem to have a couple of significant advantages.

1. They can ascend much steeper gradients, as much a 10%. This could be very significant, particularly in the the Western states.

2. Track maintenance is greatly reduced compared with railway track for high speed operation. Apparently, the Central Japan Railway, who operate the Shinkansen bullet trains, have a crew of 3,000 workers who do nothing but overhaul and check a different 12 mile section of the track - every night!

(Above information from May 2010 Scientific American)
 

Offline LeeE

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« Reply #23 on: 26/04/2010 17:48:42 »
I'm surprised that maglevs can handle inclines > 1:10, but I do believe that ~1:10 is the maximum gradient that steel wheels can operate on before they start slipping (that is to say that a steel wheel will slide on a 1:10 gradient steel rail regardless of its loading or the conditions of the wheel and/or the rail, so even just a locomotive on its own could not ascend such a slope, nor hold its position on such a slope with its brakes).
 

Offline Geezer

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« Reply #24 on: 26/04/2010 18:12:32 »
I was quite surprised to learn that too. It could make a huge difference in the cost of construction - I think railways are typically less than 3%. Course, the maglev track cost is a lot higher, so it's an interesting set of tradeoffs.

It would really make a lot of sense between LA and San Francisco. It's too far to drive but it makes no sense to fly either, and the US is really running out of airspace in the metro areas.
 

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« Reply #24 on: 26/04/2010 18:12:32 »

 

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