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Transition Curve Track Type.


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A good layout includes transition curves not just hard changes in radius.

The only way to do that at present is to add in lots of little curves of increasing radii... which isn't pretty.

It would be nice if either.....

1. ) We had a new track type that automatically generated the right spiral from a few user entered values,... OR

2. ) Flexible tracks automatically generated the appropriate spiral...


Maybe using something like this formula...
though there are others



Transition Curve.jpg

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This I did with 15 sequential 1 degree tracks, radius growth 15% per track, from a 150 degree bend to the straight parts...




BUT.. I would not want to repeat that for every bend on my layout, and the final Y position is arbitrary with that generalized method..

Edited by trevor
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Red it the transition curve, Green is flex on the same setup.
Note the "KNEE" in the flex track.

Actually the Flex detours left before going right




Detour left


lawsuit maker....


For those that don't know... transition curves stop the passengers from smashing into the walls and windows when the curve starts (Sudden change in lateral force) and reduces the amount of vomit the cleaners need to remove daily.............. they also reduce the wear on the wheels and rails and derailments.

Formulas for real rail are complicated, but the basic intent is to design the curve so the centrifugal force increases linearly or by some curve from zero to the max in the bend. Then for high speed rail you also tilt the tracks by another formula so you balance the centre of gravity of the vehicles and people in them. If you do it right, the people feel heavier but should not feel any (or much) leaning (left right) forces at the rated track speed. Interestingly, it is found that best practice requires that you leave some remaining lateral force. It was discovered that nausea ensued if you didn't because what passengers could see out of the windows did not match what their ears and muscles were telling them.


This great U-tube video shows it done well... Note the way the bends "wind" in and out and how the train tilts going through them...



This one is worth the time too...


Interestingly the issue with the above is that lines are designed for a certain speed. Trains passing over them at lower speeds can provide a great deal of discomfort for the passengers.

I recall one particular station in Aberdour in Scotland that is located on a bend on the main line. In the 70s the rails were canted to accommodate the High Speed Train (HST https://en.wikipedia.org/wiki/InterCity_125). Riding on that you didn't even notice you had gone through the station. However, when stopping there on the slower commuter train you definitely felt like you were about to slide out of  your seat. (Sorry about the watermarked pic... best I could find that shows the coach tilt.)

For this reason many high speed lines are separated from the regular lines.

The Brits, doing what Brits do, pioneered a different approach around the same time.

They figured, rather than tilt the rails, why not tilt the train. And so the Advanced Passenger Train (APT https://en.wikipedia.org/wiki/Advanced_Passenger_Train) was born.

Technology wise it was, unfortunately,  a little far ahead of its time back then and had a rather embarrassing launch from which it never really recovered. However, the technology still exists today as the Pendolino class in various countries including the UK. https://en.wikipedia.org/wiki/Pendolino



Edited by trevor
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Hallo Neo,

Am 12.1.2017 um 13:58 schrieb trevor:

Red it the transition curve, Green is flex on the same setup.
Note the "KNEE" in the flex track.

Actually the Flex detours left before going right

die beiden Kurven unterscheiden sich nach meinem Verständnis folgendermaßen:

Trevor möchte, daß der rote Übergangsbogen ausgehend von der Geraden (R=unendlich) bis zum Radius des Kreisbogens so verlaufen soll, daß ein Radiusanschluß ohne Krümmungssprung erreicht wird.

Das grüne Flexgleis besteht aus mindestens 2 Kurvenelementen

  1. von der Geraden bis zum Punkt der größten Krümmung
  2. vom Punkt der größten Krümmung bis zum Bogenanfang des Anschlußkreises. Dabei schließt das Flexgleis aber an den Radius so an als wäre die Kurve eine Gerde - will sagen - das Flexgleis hat am Ende wieder den (Radius=unendlich). Da bedeutet, daß der Radiensprung immer noch vorhanden ist.
  3. Das Knie rührt daher, daß der erreichbare seitliche Versatz zwischen Gerade und Kurve nicht genau stimmt und durch daß Flexgleis ausgeglichen wurde.

Ich vermute, daß Trevor sich eine Lösung wünscht, in dem die halbe Flexgleisverformung (nach Pkt.1) angeboten würde.

Bezüglich echter Übergangsbögen und der Überhöhung melde ich mich noch mal

meint Henry


The two curves differ as follows:
Trevor would like the red transition arc to extend from the straight line (R = infinity) to the radius of the circular arc so that a radius connection without a curvature jump is achieved.

The green flex track consists of at least 2 curve elements

  1. From the straight to the point of greatest curvature
  2. From the point of greatest curvature to the beginning of the arc of the connection circle. In this case, however, the flexure adjoins the radius as if the curve were a blatant - the flexion finally has the (radius = infinity). This means that the radii jump is still present.
  3. The knee is due to the fact that the achievable lateral displacement between the straight line and the curve is not exactly correct and is compensated by the flexure track.

I suspect that Trevor would like a solution in which the half flexural deflection (according to point 1) would be offered.

Regarding real transitional arcs and tilting the rails I report again 



Edited by Henry
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da es mich interessiert hat, habe ich mir ein kleines Programm geschrieben, mit dem ich im MBS die Linie einer Klothoide (mit einer Serie von Objekten) darstellen kann... das Beispiel zeigt den Unterschied Klothoide - Flexigleis für R=424,6 und einen Winkel von 45 Grad.

because I was interested, I wrote a small program with which I can represent in the MBS the line of a clothoid (with a series of objects) ... the example shows the difference clothoid - flexigleis for R = 424,6 and an angle of 45 degrees....
Bild 1.jpg


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Hallo EASY, hello Trevor,

solch ein "sanftes" Flexgleis hätte ich auch schon des öfteren gebrauchen können. Ich habe dann den "Löwenanteil" des Bereichs, den ursprünglich das Flexgleis überbrücken sollte, mit Gleisstücken mit festem Radius belegt, nur um die beim gelben Gleis sirchbare "Ecke" (oder "Ausbeulung" zu verhindern. Damit konnte ich dann zwar halbwegs akzeptable Ergebnisse erzielen. Doch der Aufwanf war natürlich bedeutend höher als nur das Flexgleis "einzuschieben".

Such a "soft" flex track I had often missed. So, the "main part" of the track portion, which the flex track should originally fill, I have filled with fix radius pieces to avoid the "corner" (or "buckling") seen at the yellow colored track above. I got nearly acceptable results with this method. But it was more expensive of course than simply "pushing in" the flex track.

Viele Grüße / many greetings

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Hallo EASY,

ich rufe dich morgen abend an --- da bin ich extrem auf Deine Vorgaben / Annahmen neuierig ... (wegen des sanften Flexgleises)

HG, Henry


I call you tomorrow evening --- because I am extremely keen on your specifications / assumptions ... (because of the soft Flextrack)


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