Using the make transition function
to create a non-concentric running loop

1. Here first is our requirement, based on Roderic Cameron's original plan. The running line labelled a has had diamond-crossings (which are intended to be converted to slips) inserted at b and c, and it is required to fit a wide running loop between them. (They could have been ordinary turnouts without affecting this sequence.)

As an opening gambit I added a short length of straight track at d to start the loop. It could equally well have been curved slightly, or a different length, according to the required shape of the loop. A similar length could be added at c if the design requires it.

2. Using the peg and notch functions I added a fixed curve of large radius labelled e, and put it on the background. Then I created a smaller radius f as the current template. I changed these to centre-line only templates for clarity (geometry > track centre-lines only menu item). Both these tracks are dummy templates, created only as an aid to design. Our intention is to replace them with a single transition curve.  

3. In this screen I am using the F6 curving mouse action to adjust the radius of curve f. For the make transition function to work, it is necessary that these two curves do not quite touch or intersect. It is also necessary that even if both of them were extended to form complete circles, they would still not touch or intersect. It is usually possible to judge this intuitively without needing to zoom out and extend them to full circles to find out. Clearly if the inner radius is significantly smaller than the outer radius, as in this view, this condition will be fulfilled - the smaller radius circle would be completely contained within the larger one.

Strictly speaking it is permissible for them to just touch, but only if they do so with much greater precision than can be achieved by mouse action on the screen. The result would be a zero-length transition at the tangent point.

4. I then clicked on the background template (arrowed blue), and on its pop-up menu I clicked the peg/align tools > make transition curve from current template > try left-hand first menu item. But the result after a short wait was not what we are looking for. It is certainly a transition curve linking the two dummy templates, but it is the wrong hand. This dialog window then appeared, so I clicked the try opposite hand instead option.

5. The result was the transition curve which we wanted. It is now the current template and the previous current template (curve f) has been put on the background. Note that although Templot has calculated the position and length of the transition zone, it has added only a nominal length (1 chain or 66ft scale) beyond the transition zone at each end. This is not enough to completely replace the dummy templates, and we need to either extend the transition template at each end (arrowed 1 and 2), or shorten the dummy templates to fill the remaining gaps.

For a more detailed explanation of what you are seeing, click about transition curves.

6. I decided to extend the transition template, so I deleted the two dummy templates as no longer needed. Also in this view I have restored the rails (untick the geometry > track centre-lines only menu item).

7. Here I have used the F4 overall length mouse action to extend the transition template to the right.

8. And in this close-up you can see that it matches exactly.

 Don't use the peg and notch functions to make this join, it is important that the calculated alignment of the transition zone is not disturbed. Simply extend the track to join or overlap by a fraction.

9. Now we need to repeat the process to fill the slight gap at the other end. So I clicked the peg indicator (arrowed) to swap the fixing peg to the opposite end of the template.

10. And again used the F4 overall length mouse action to extend the template into the gap.

In other circumstances the new template may be too long, and need to be shortened. If it is judged that the transition zone is too long, we could cancel this curve and try again with a reduced space between the original templates by changing the radius of one or the other slightly (as in view 3 above).

If one of the original templates was itself a transition template, the new transition will be matched to it according to the position of its fixing peg. If the peg is within the transition zone, the original template will need shortening to that exact position to obtain a proper match with the new template (use the do > snap to peg menu item).

11. Here is the finished loop, comprising a short straight and a long transition template. Now we can wipe this back to current, insert a turnout, make splits and insert further turnouts, in the usual way. Or we can treat this as a further dummy template, and use the align current template over background template functions to replace it with one or more previously designed custom turnouts, for example.

For further notes on using the make transition function, click the peg/align tools > make transition curve from current template > ? make transition - help menu item.


transition curves

Templot Companion


1-Dec-02 ©