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working with return curves and parallel crossings

including some notes on using transition curves and slewing

a step-by-step tutorial sequence

 This is a mini-tutorial. Please refer to the general notes about using these tutorial pages at the beginning of the the track plan tutorial.

There is a text-only version of this tutorial which can be printed out for convenience.

A return curve is a curved length of plain track which returns the diverging road of a turnout back towards the main road, so that the two roads become parallel or concentric (marked with yellow arrows in screen A, templates 1 and 2).

A return curve is often employed to form the end of a running loop, or to make a junction between double and single track, or to access an island platform.

screen A:

Screen A shows six different return curves, each attached to a left-hand B-6 turnout in S4/P4 gauge. The main road in each case is straight for simplicity in the diagram, but it could equally well have been curved on a fixed radius (but not in all cases on a transition curve or containing a slew - see later).

There are four different ways to create a return curve in Templot:

  1. Using the tools > make return curve function.

  2. Setting the parallel crossing type of V-crossing for the turnout.

  3. Do-it-yourself using the peg and notch functions to align a separate transition curve plain track template.

  4. Do-it-yourself using the slew functions to modify the end of the parallel loop track.

Each of these methods has its pros and cons, summarised in the table below:

return curves in Templot :   tools > make
return curve
do it yourself
( transition )
do it yourself
( slew )
the return curve is a separate
plain track template which can
have a turnout inserted, and over
which another can be aligned.
can be
combined with
the loop track
the main road can be
on a transition curve,
or contain a slew.
the return curve radius
and length of intervening
straight can be adjusted.
no alignment
is variable
the method can be used with
a curved crossing type
of V-crossing.
yes, but not
the method can be used with
a generic crossing type
of V-crossing.
the method is quick to use.
not very no

Method 1.  tools > make return curve :

This function creates the return curve as a separate template which is a length of plain track of fixed radius. The radius is always set to the maximum which will fit between the turnout and the centre-line of the adjacent track on the turnout-side (TS), the spacing of which can first be set by clicking the geometry > adjacent centres... menu item.

In screen A, template 1 shows the result of using the tools > make return curve menu item when the adjacent track spacing is set to its pre-set startup setting, which for this S4/P4 gauge template is 44.67 mm centre-to-centre (11' 2" scale for 6' way between the inner rails).

For template 2, I changed the TS adjacent track spacing to a much wider setting, 100 mm centre-to-centre. Using make return curve then produced a much longer and easier return curve.

Because the return curve template is a separate plain track template, independent of the turnout, it can be modified and adjusted like any other, have a turnout inserted in it (track > insert turnout in plain track menu item), or after copying to the background have a new current template aligned over it.

There is however a limitation on the use of the make return curve function. It is not available when the main road of the turnout is on a transition curve, or contains a slew. In these cases, one of the other return curve methods must be used.

Method 2.  parallel crossing :

Templates 3, 4, 5, and 6 in screen A show return curves which are part of a turnout which has a parallel crossing. These are all single templates, and the return curve is an integral part of them, not a separate template.

In template 3 the adjacent track centres have been set at 44.67 mm, and in template 4 the adjacent centres have been increased to 100 mm. You can see that in both cases the return curve radius is the same, corresponding approximately to the radius of the turnout and similar to that in template 1. The difference in template 4 is that a length of straight has been introduced to make up the extra track spacing.

These two templates 3 and 4 illustrate the most common way of using a parallel crossing.

But in templates 5 and 6, both the return curve radius and the length of intervening straight are different, and this is done by changing the parallel crossing track centres dimension independently of the adjacent track centres dimension. The screens below show how these settings are used. 

screen B:

For a parallel crossing, click the track > crossing... menu item, and then select parallel crossing in the list on the crossing selector window which appears (screen B). Change the parallel track centres option to other... so that we can set this dimension for the parallel crossing independently of the current adjacent track spacing dimension. Then click OK.

screen C:

On the data-entry form in screen C, set the turnout road track centres to, say, 100 mm. Note that this is not the same thing as the adjacent track centres setting, which remains as previously set. This dimension applies only to parallel crossings, and only when the other... option is selected (screen B).

screen D1:

Screen D1 shows the result. The turnout now has a return curve linking the turnout road to parallel track at 100 mm centres. Because we haven't changed the adjacent track centres dimension (which remains at 44.67 mm), the straight section between the turnout and the return curve is the same length as it would have been if the parallel track centres had been set to that dimension. The additional spacing out to 100 mm centres is made up by a long easy return curve of 5441.34 mm radius. These dimensions are arrowed in screen D1.

Checking the track menu (or the right-click menu) we see that the length free option is selected. This means that despite appearances there is no exit track (or approach track) on this template, which is set to the free overall length. The free overall length of a parallel crossing template is set to the end of the return curve, i.e. the position at which both tracks become parallel (or concentric in the case of a curved main road) (arrowed). The ends of the tracks at this position are the MRP (Main-road Return Point), and TRP (Turnout-road Return Point), and the fixing peg can be set at these positions if desired (adjust > set peg options > menu items).

This template is now the same as template 5 in screen A.

screen D2:

If the F4 overall length mouse action is now used, exit tracks can be added to both tracks, or both can be shortened back, but it is not possible for the lengths of the two tracks to differ.

In screen D2, if the F6 curving mouse action is tried, you will see that both tracks are adjusted accordingly, and it is possible to set a main road curving radius such that the return curve is actually straight, or curved in the same direction as the main road, positive or negative (showing negative for this left-hand turnout). I have set the peg at the TRP position. (The peg indicator shows N because there is no keyboard shortcut for this position).

Remember that this is all one single template (which could also have some approach track added). It could also be on a transition curve in the main road, or contain a slew.

screen E:

Now in screen E click the geometry > adjacent centres... menu item,

screen F:

and in screen F set the turnout-side adjacent track centres to 100 mm, matching the parallel track centres entered previously. The main-side dimension is not used here and can remain unchanged.

screen G:

Screen G shows the result. The return curve is much shortened by having a longer intervening straight section and a much smaller radius. Compare the arrowed dimensions with those in screen D1.

This template is now the same as template 4 in screen A.

screen H:

Screen H explains what is happening. I have drawn a yellow line at 50 mm from the main road centre. This would be the centre-line between the tracks if we created double track at the present adjacent track spacing dimension of 100 mm. In generating the straight portion for a return curve in a parallel crossing, Templot always sets the length of this straight so that it is equally disposed about the centre-line between adjacent tracks, calculated from the fine-point of the turnout. In other words, the two blue arrows in screen H are equal in length.

Note carefully that the dimension used to make this calculation is the current adjacent track spacing setting, not the actual parallel track spacing which is being used for the loop track. Here these two dimensions are the same (100 mm), but as we have seen in screens D1 and D2 they need not be so.

When the adjacent track spacing is less than the parallel track spacing (as in screen D1), a longer, easier return curve is produced, with a shorter intervening straight section.

When the adjacent track spacing is the same as the parallel track spacing, and the main road is straight (as here in screen G), the radius of the return curve is approximately the same as that of the turnout. (Approximately, because the turnout road within a turnout is not a single curve.)  

When the adjacent track spacing is greater than the parallel track spacing, a tighter return curve radius is produced. This can easily be taken to extremes, as shown in template 6 on screen A, where the adjacent track centres exceed the parallel track centres by only 15 mm, but the curve produced is unusable.

Summarising for a parallel crossing :

To change the length of the intervening straight section, change the adjacent track centres dimension (geometry > adjacent centres... menu item).

To change the radius of the curve section, change the parallel crossing track centres dimension (track > crossing... menu item, then other... option button).

Method 3. Do-it-yourself return curve (using a transition curve) :

In the next part of this tutorial we shall create a return curve which is similar to that for a parallel crossing, but as a separate template which can be modified and adjusted in the same way as any other template.

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© revised 15-12-00.

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