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

In this 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.


screen 1:

Our object in this part of the tutorial is to create a short return curve template in S4/P4 for this B-6 turnout, with the track spacing at 100 mm centre-to-centre. For simplicity in the diagrams I have assumed a straight template (main road is straight), but these methods work equally well for a fixed-radius curve or a transition in the main road.

In screen 1, extend a reasonable length of exit track (F4 overall length mouse action), and then put the peg at the TVJP (Turnout-road Vee Joint Position) (CTRL-6), as shown. Make a note of the minimum radius in this turnout (1167 mm, arrowed). We shall later need to set a radius for the return curve, and we may as well use this figure.

Click the geometry > adjacent centres... menu item,

screen 2:

and then set 100 mm for the turnout-side adjacent track centre-to-centre spacing (screen 2).


screen 3:

In screen 3, make a length of double track on the turnout side (tools > make double track TS menu item),

screen 4:

and put it on the background (store & background menu item, here on the right-click menu) (screen 4). This template is needed temporarily as an aid to aligning the return curve.


screen 5:

The basis of our return curve will be a transition curve template, so click the geometry > curved (transition) > easement from straight menu item as shown in screen 5 (in which the current template remains hidden after copying to the background).


screen 6:

So press F12 to see it again, looking like this (screen 6). If what you are seeing is unfamiliar, please click transition curves and read the notes about them before proceeding.

Now click the geometry > curving data... menu item,

screen 7:

and enter these dimensions (screen 7). The initial radius is already straight (pre-set), and we have decided to use 1167 mm as the final radius for the return curve section. The length along initial radius is an arbitrary guess at 150 mm (we are about to change it). The length along transition zone is set to zero for this template, making the straight section tangential with a simple fixed-radius return curve section.

All these dimensions can be changed to suit different requirements as you wish. Click OK to continue.


screen 8:

Screen 8 shows the result. Because we set the transition zone length to zero between the straight and the curved section, the transition start and end markers are superimposed, as shown (arrowed).

Now click anywhere on the turnout template to bring up its menu, and click the peg / align current > notch under peg and shift current menu item.


screen 9:

The result in screen 9 is that the current template is pegged onto the background turnout template. (We previously set the turnout's peg at the TVJP position in readiness, see screen 1.)

Now we want to change the length of the straight section (shown with blue arrows) until the curved section is exactly tangential with the temporary adjacent track. There is no automatic function to do this, but by zooming in close we can do it very accurately by eye on the screen. Select the adjust > mouse actions: current > move transition start (SHIFT+CTRL-F3) mouse action.


screen 10:

Then in screen 10 adjust the position of the transition markers (arrowed 1) until the curve is approximately tangential with the adjacent track (arrowed 2).


screen 11:

Zoom in closer (screen 11), and shorten the template (F4 overall length mouse action) until there is a more convenient overlap from the tangent point of, say, three sleepers-length (arrowed blue).


screen 12:

Zoom in much closer (screen 12), and note the remaining discrepancy between the track alignments (arrowed). We need to make a further slight adjustment to the transition start, but before doing so it will be easier to see what is happening if we remove the timbering from current template.

There are several ways to do this, a convenient one is the pad > screen refresh options > skeleton mouse draw menu option, which draws the current template in skeleton form during mouse actions. This option can be toggled on and off at any time by pressing its single-key alternative keyboard shortcut, which is the ; (semi-colon) key. There are many more of these single-key alternatives, they are shown on the Function Key Chart (help > print F key chart menu item to print it out).


screen 13:

Now in screen 13 make the final adjustment to the transition curve (SHIFT+CTRL-F3 transition start mouse action again), until the rail-edges and track centre-lines are exactly aligned (blue arrows). Notice that it is not necessary to have the transition markers actually visible while adjusting their position.


screen 14:

Then in screen 14 make a further adjustment to the overall length, terminating the return curve at the approximate position of the tangent point. The exact position is not important, and at this degree of magnification can be easily judged by eye.


screen 15:

Screen 15 shows the finished return curve template, which we can now put on the background.


screen 16:

Now we can shorten or delete the temporary adjacent track as necessary. First delete to current (screen 16),


screen 17:

then swap the peg to the opposite end by clicking the peg indicator (arrowed) (or CTRL-1), and shorten the template as necessary using the F4 overall length mouse action (screen 17). A slight gap or overlap is of no consequence.


screen 18:

 The end of the return curve was set by eye only. If it is desired to peg further templates at this position, for the maximum accuracy it is better to set the notch from the adjacent track template rather than from the return curve. I therefore swapped the peg back to this datum end (CTRL-0) before putting the notch under it (DIVIDE key on the number pad). For more information about the peg & notch functions, click peg & notch.

You can see that I overlapped the templates slightly, the vertical black line is the end marker for the return curve template (screen 18).


screen 19:

Screen 19 shows where we have reached. This return curve is essentially the same as the one which we previously created using a parallel crossing, see screen G and template 4 in screen A. The important difference is that this return curve is now a separate template in its own right, which means that we can use all the usual Templot functions on it.


screen 20:

As an example in screen 20 I did delete to current, then track > insert turnout in plain track, then track > hand > invert handing, and CTRL-F9 maintain length mouse action. Possibly also tools > swap current end-for-end. None of this is possible when the return curve is part of a parallel crossing (as in screen G).


screen 21:

Now in screen 21 I have repeated the whole exercise for a turnout having a negative radius of 4000 mm in the main road. As you can see, the original return curve needs further adjustment.


screen 22:

In screen 22 I first lengthened the template (F4 overall length), and then shortened the length of the straight section (SHIFT+CTRL-F3 move transition start) until the return curve was again tangential with the temporary adjacent track.

But unless space is tight, there are other adjustments which we could make. For example, it is possible to get an easier curve by putting the straight section on a gentle curve, or by introducing a finite (non-zero) length of transition zone. We might also decide to change the radius of curve section.

We can now immediately adjust any or all of these four transition settings by mouse action, or enter some trial dimensions directly as a starting-point.


screen 23:

I chose the latter course and clicked the geometry > curving data... menu item to bring up the data-entry form (screen 23), and then entered these dimensions:

The former straight section I changed to an initial radius for the transition of 4000 mm, matching the radius at this point in the turnout-road (for a regular crossing as here, this is curved to the same radius as the main-road).

For the return curve I entered an easier final radius of 1500 mm.

I then guessed trial dimensions of 120 mm for the length along the initial radius, and 240 mm for the length of the transition zone.


screen 24:

Screen 24 shows the result, an easier return curve which is not very much longer than before. It needs a little further adjustment. To do this there is a choice of four transition curve adjustments:

  1. extend the length of initial radius section (SHIFT+CTRL-F3 move transition start mouse action)
  2. extend the length of the transition zone (SHIFT+CTRL-F4 transition length mouse action)
  3. increase the initial radius (SHIFT+CTRL-F1 1st transition radius then F6 curving mouse action)
  4. increase the final radius (SHIFT+CTRL-F2 2nd transition radius then F6 curving mouse action)

I chose the second option and adjusted the length of the transition zone (screen 24).

 Options 1 and 2 mouse actions can be more conveniently selected using the single-key shortcuts, which are the [ and ] (square brackets) keys respectively.

 When using the F6 curving mouse action to adjust the radius, clicking the hollow-triangle mouse action symbol on the mouse action panel toggles between the 1st (initial) and 2nd (final) radius. Or these can also be conveniently selected using the single-key shortcuts, which in this case are the - (minus) and = (equals) keys respectively. These keys can be pressed while the mouse action is in force. 


screen 25:

Screen 25 shows the final result, after zooming in as before (screens 11-14) to make the final adjustments to the alignment and overall length.


Method 4. Do-it-yourself return curve (using the slew functions) :

In the final part of this tutorial we shall create a return curve by means of a completely different method, which is to slew the end of the loop track into line with the turnout.

This involves pushing the slew functions somewhat beyond their intended purpose as a means of "tweaking" alignments, but if done carefully can produce an acceptable return curve. The advantages over the other methods are that the length of the return curve can easily be set initially to fit the available space, and the return curve can be combined in one template with the loop track, which is useful when you want to insert a turnout which bridges the end of the return curve.


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© revised 6-12-00.


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