Movable point diamonds are used only for very shallow angle crossings. Tree refers to potholes. As the angle gets very shallow, the flangeway gap in the direction of travel becomes larger and so does the resulting impact force on the frog point. The possibility of a wheel picking the point, especially if damaged, also becomes greater. So, movable points are required.
With movable points, however, they have to be interlocked into a signal system to ensure they are fully closed for the route to be used, and of course some sort of operating mechanism is needed. They are used only when there is no alternative. Regular diamonds, while still somewhat of a maintenance headache, have the advantage of simplicity.
I don't know what angle marks the normal limit for a regular diamond but would guess it to be around 8 degrees. (Any of our experts know for sure?)Fortunately few diamond crossings are even close to that, often closer to 90 degrees.
The movable point diamond is most widely seen as the central part of double (and single) slip switches in passenger terminal throats. The additional complexity of the movable point frogs is manageable in that specialized environment.
John
cx500 Movable point diamonds are used only for very shallow angle crossings. Tree refers to potholes. As the angle gets very shallow, the flangeway gap in the direction of travel becomes larger and so does the resulting impact force on the frog point. The possibility of a wheel picking the point, especially if damaged, also becomes greater. So, movable points are required. With movable points, however, they have to be interlocked into a signal system to ensure they are fully closed for the route to be used, and of course some sort of operating mechanism is needed. They are used only when there is no alternative. Regular diamonds, while still somewhat of a maintenance headache, have the advantage of simplicity. I don't know what angle marks the normal limit for a regular diamond but would guess it to be around 8 degrees. (Any of our experts know for sure?)Fortunately few diamond crossings are even close to that, often closer to 90 degrees. The movable point diamond is most widely seen as the central part of double (and single) slip switches in passenger terminal throats. The additional complexity of the movable point frogs is manageable in that specialized environment. John
AREMA recommends movable points at the interior frogs for any crossing 9 degrees 30' or less.
cx500 Movable point diamonds are used only for very shallow angle crossings. Tree refers to potholes. As the angle gets very shallow, the flangeway gap in the direction of travel becomes larger and so does the resulting impact force on the frog point. The possibility of a wheel picking the point, especially if damaged, also becomes greater. So, movable points are required.
Euclid If this is the proper interpretation, I do not see why a shallow angle would cause more wheel impact (pothole effect) than a less shallow angle. The most severe impact occurs with a 90 degree crossing because the wheels cross the opposing flangeway at a right angle. It would be like wheels crossing a rail joint where the rails had pulled apart leaving a gap of about 1.5 inches. With a shallow angle frog, the wheel does not drop down from a loss of support as far as it does with a 90 degree crossing. With a shallow angle frog, rail bearing surface drops away gradually. Before it completely disappears, it begins to be re-introduced by running surface on the other side of the gap. The wheel crosses the gap obliquely, so as it gradually loses support on one side of the gap, it gradually picks up support on the other side of the gap.
We'll quote from Hay's Railroad Engineering textbook.
"as the crossing angle diminishes, the length of the frog throat (flange way ed.) in the center frogs that the wheel must jump increases. A point is reached where this distance becomes dangerously wide. Movable point frogs are then installed"
don't know how to make it any clearer than that.
Over and out.
In a shallow angle crossing we have two very different pairs of frogs, outer ones with an acute angle and the other pair with an obtuse angle. For the former, guard rails are used to provide additional guidance for the wheel flanges, as seen on many turnouts. But at the center, obtuse angle frogs, guard rails are not feasible. And that becomes a safety concern.
The pothole effect exists with all fixed frogs, including those in turnouts. It can be eased by replacing a frog with movable points, but that just replaces one maintenance headache with another. Which is least bad is entirely dependent on the local situation. Just another case of a problem with no desirable answer.
EuclidWith a shallow angle frog, rail bearing surface drops away gradually. Before it completely disappears, it begins to be re-introduced by running surface on the other side of the gap. The wheel crosses the gap obliquely, so as it gradually loses support on one side of the gap, it gradually picks up support on the other side of the gap.
This reminds me a little of how Angus Sinclair used to complain about the little arrows that showed how the steam was supposed to flow through 'patent improved' valves and devices. He would say that the steam would obstinately refuse to be convinced by the inventor that it should follow the arrows, and persist in going where it wanted.
Here we have a number of positive statements about how shallow-angle frogs work ... none of which are particularly true for what happens with actual frogs in actual track being traversed by actual wheels. I confess to being a little awed by the audacity with which these have been presented one after the other as if they were objective engineering facts.
I suppose that if you had wheels with negligible taper, with the tread width of the most extreme 'poley' blind drivers, you might get to the point where you'd span the width of the flangeway in the frog as noted to a sufficient extent that there was no flange bearing. But that certainly doesn't apply to normal car wheels on the kinds of frog we are discussing!
At least we're not hearing about an approach that would make flange bearing through the gap the means of eliminating drop and jar. There are all sorts of problems with doing that, even if transit systems could make it work under some circumstances.
cx500 In a shallow angle crossing we have two very different pairs of frogs, outer ones with an acute angle and the other pair with an obtuse angle. For the former, guard rails are used to provide additional guidance for the wheel flanges, as seen on many turnouts. But at the center, obtuse angle frogs, guard rails are not feasible. And that becomes a safety concern. The pothole effect exists with all fixed frogs, including those in turnouts. It can be eased by replacing a frog with movable points, but that just replaces one maintenance headache with another. Which is least bad is entirely dependent on the local situation. Just another case of a problem with no desirable answer. John
Thanks John,
That is exactly the answer I just received from the editor of Model Railroad Hobbyist Magazine in relation to the previously linked article on building a model of this type of crossing.
The inner frogs are not capable of providing a guardrail due to the frog geometry. The outer frogs are capable of providing a guardrail. So as the angle becomes shallower, the lack of guardrails on the inner frogs forces them to be replaced by switch points.
Overmod Euclid With a shallow angle frog, rail bearing surface drops away gradually. Before it completely disappears, it begins to be re-introduced by running surface on the other side of the gap. The wheel crosses the gap obliquely, so as it gradually loses support on one side of the gap, it gradually picks up support on the other side of the gap. This reminds me a little of how Angus Sinclair used to complain about the little arrows that showed how the steam was supposed to flow through 'patent improved' valves and devices. He would say that the steam would obstinately refuse to be convinced by the inventor that it should follow the arrows, and persist in going where it wanted. Here we have a number of positive statements about how shallow-angle frogs work ... none of which are particularly true for what happens with actual frogs in actual track being traversed by actual wheels. I confess to being a little awed by the audacity with which these have been presented one after the other as if they were objective engineering facts. I suppose that if you had wheels with negligible taper, with the tread width of the most extreme 'poley' blind drivers, you might get to the point where you'd span the width of the flangeway in the frog as noted to a sufficient extent that there was no flange bearing. But that certainly doesn't apply to normal car wheels on the kinds of frog we are discussing! At least we're not hearing about an approach that would make flange bearing through the gap the means of eliminating drop and jar. There are all sorts of problems with doing that, even if transit systems could make it work under some circumstances.
Euclid With a shallow angle frog, rail bearing surface drops away gradually. Before it completely disappears, it begins to be re-introduced by running surface on the other side of the gap. The wheel crosses the gap obliquely, so as it gradually loses support on one side of the gap, it gradually picks up support on the other side of the gap.
Always fun to see the casual obsevers know more than the professionals. Seems to be a few of them about. Guess the professionals are just plain dumb. For a short while all the inventors that approached the industry or FRA with an idea got referred to me, was glad to get out of that role.
i actually had one guy that wanted to build two track wide trains that utilize the double tracks that exist "everywhere" right? The cars would have 4 trucks that would slide sideways to accommodate varying track centers. The guy was a disabled veteran and I felt somewhat sorry for him spending what I assumed were scarce resources on a concept that had little chance. I told him to back off as I doubted he'Ed get anywhere with it. He wrote a nasty gram to my boss's, boss's boss complaining about how closed minded I was. Yup glad I passed that assignment on to others.
Buslisti actually had one guy that wanted to build two track wide trains that utilize the double tracks that exist "everywhere" right? The cars would have 4 trucks that would slide sideways to accommodate varying track centers.
Was this by any chance related to Trains publishing "The Case for the Double-Track Train" in the '70s? With the proof-of-concept demonstration pictures that I believe used HO gauge tracks?
Buslist AREMA recommends movable points at the interior frogs for any crossing 9 degrees 30' or less.
- Paul North.
Shooshie This now ancient conversation needs a sort of definitive tie-up. I'm going to go with the video posted by Euclid above, which I'll repost here: https://www.youtube.com/watch?v=ZuR5QTlfOzk It's at 4:33 that the narrator calls it a "Slip Diamond." I don't like the word "slip" confusing it with single or double slip switches, but I'll go with the name anyway until I hear better. The problem is the usage of that word "slip." A slip implies that there is an alternate route, not just a crossing. If track AB crosses track YZ, it normally would have a diamond with 4 frogs, often with replaceable elements for repairing the wear encountered from thousands of wheels bumping over it at any speed. However a crossing with a slip, called a single slip switch, would still allow AB to cross YZ, but would provide for an alternate route, AZ. A DOUBLE Slip would allow not only the alternate route AZ, but another alternate route, YB. The Slip Diamond Crossing sounds like it would have alternate routes, but it does not. If you're coming from point A, the only place you can go is point B, or get derailed. Likewise, from point Y you can only go to point Z. So the word 'slip' seems ambiguously applied here, and yet I have no other word for it. Actually, I do. I'd consider calling it a movable point diamond crossing. It's not perfect, but it doesn't imply there are alternate routes, and it DOES describe exactly what's going on there. However, it appears that "point" is used outside of North America to refer to "frog," so a movable point becomes a movable frog. That muddies up our definition, so that may not work. I guess we're stuck with "Slip Diamond Crossing" for now, but everyone who reads it will think "double slip switch." [or single] And we've certainly established that it's NOT THAT!
This now ancient conversation needs a sort of definitive tie-up. I'm going to go with the video posted by Euclid above, which I'll repost here:
https://www.youtube.com/watch?v=ZuR5QTlfOzk
It's at 4:33 that the narrator calls it a "Slip Diamond." I don't like the word "slip" confusing it with single or double slip switches, but I'll go with the name anyway until I hear better.
The problem is the usage of that word "slip." A slip implies that there is an alternate route, not just a crossing. If track AB crosses track YZ, it normally would have a diamond with 4 frogs, often with replaceable elements for repairing the wear encountered from thousands of wheels bumping over it at any speed. However a crossing with a slip, called a single slip switch, would still allow AB to cross YZ, but would provide for an alternate route, AZ. A DOUBLE Slip would allow not only the alternate route AZ, but another alternate route, YB.
The Slip Diamond Crossing sounds like it would have alternate routes, but it does not. If you're coming from point A, the only place you can go is point B, or get derailed. Likewise, from point Y you can only go to point Z. So the word 'slip' seems ambiguously applied here, and yet I have no other word for it.
Actually, I do. I'd consider calling it a movable point diamond crossing. It's not perfect, but it doesn't imply there are alternate routes, and it DOES describe exactly what's going on there. However, it appears that "point" is used outside of North America to refer to "frog," so a movable point becomes a movable frog. That muddies up our definition, so that may not work.
I guess we're stuck with "Slip Diamond Crossing" for now, but everyone who reads it will think "double slip switch." [or single] And we've certainly established that it's NOT THAT!
I think people are misusing terminology (or perhaps mispronouncing combination words).
Two tracks that cross at other than a right angle: "Diamond Crossing"
Crossing of this type with slip feature is commonly called just "slip" (not slip switch, as in US practice) - qualified as 'single slip' or 'double slip' (with the word 'turnout' or 'crossing' understood)
A "slip diamond" by definition would be at least single-slip in its function (regardless of the picture). It would NOT just be a fancy name for movable-point crossing that reduces shock in traversing. There is, therefore, no 'contradiction in terms' regarding the use of the word 'slip' -- when it is applied in proper terminology. Here is a link that might help resolve this business.
EuclidBut I don’t find any dictionary definition for “slip” that ties it to railroad track features.
You probably won't -- except in a specialized dictionary like the one C. Hamilton Ellis edited, or perhaps in a dictionary that also knows what a 'slip coach' is (that's another wonderful Britannic service provision!)
I've always thought the sense was that you could 'slip' from one track route onto another, rather than following tracks through specified turnouts and such. Until very recently I thought all slip switches were bidirectional, like an operational version of the Railbox logo. That's not so; please don't chuckle at my naivete. But without the ability to 'slip', I don't consider that you have a slip switch (or slip anything, really) even if there are movable points just like switch points within a diamond that make traversing it smoother.
to find different terms used for the same parts.
Johnny
Double slip switch is a combination of a track crossing (four frogs) and two turnouts interconnected into one assembly. Single slip switch is the combination of a track crossing (four frogs) and one turnout. The slip rail is the the curved stockrail between the crossing frogs.(Double has four and the single has two...these are low speed switches with restrictive / cramped geometry and PDN and I can testify as to how much of a bear these are to build and maintain.)
Bucky, quit being so cheap and buy a REAL dictionary or at least get a Simmons Boardman Dictionary of Railway Track Terms fom Omaha.
EuclidI guess it is like asking why a frog is called a frog.
It's called that because it was reminiscent -- perhaps more in the early days of railroading -- of that part of a horse's anatomy.
EuclidI don’t doubt that there is a definition of slip switch, and maybe even parts of the switch named as “slip” parts. I am just wondering why the term “slip” is used to name these items. What slips? I guess it is like asking why a frog is called a frog.
Did a little digging and found an interesting statement - "On the Great Western Railway, they were known as double compound points, and the connections were slip roads." That can be found on this website http://mysite.du.edu/~jcalvert/railway/slip.htm
It then occured to me that I've heard the term "slip road" before - on my GPS. When it wants me to get off the freeway, it tells me to take the next "slip road."
Thus one conclusion that can be drawn is that the term "slip" doesn't refer to the parts of the switch at all - it has to do with the potential routings out of the slip switch. A single slip switch allows entry on either of two tracks, but exit on only one, the way I read it. So there is only one exit - or "slip."
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
tree68 Euclid I don’t doubt that there is a definition of slip switch, and maybe even parts of the switch named as “slip” parts. I am just wondering why the term “slip” is used to name these items. What slips? I guess it is like asking why a frog is called a frog. Did a little digging and found an interesting statement - "On the Great Western Railway, they were known as double compound points, and the connections were slip roads." That can be found on this website http://mysite.du.edu/~jcalvert/railway/slip.htm It then occured to me that I've heard the term "slip road" before - on my GPS. When it wants me to get off the freeway, it tells me to take the next "slip road." Thus one conclusion that can be drawn is that the term "slip" doesn't refer to the parts of the switch at all - it has to do with the potential routings out of the slip switch. A single slip switch allows entry on either of two tracks, but exit on only one, the way I read it. So there is only one exit - or "slip."
Euclid I don’t doubt that there is a definition of slip switch, and maybe even parts of the switch named as “slip” parts. I am just wondering why the term “slip” is used to name these items. What slips? I guess it is like asking why a frog is called a frog.
I just now noticed that the first line of my post disappeared--I do not remember it exactly, but it something like this: I have the impression that the video is produced in Great Britain, so we can expect to find different terms used for some parts.
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