Working with return curves

Page 4.

screen 42:

In screen 42 click the geometry > slew (nudge) > slew using mode 2 menu item.

screen 43:

After extending the overall length (F4), screen 43 shows the result, which is a more pronounced reverse curve effect.

screen 44:

Now in screen 44 with timbering switched off again, we repeat the process of adjusting the slew length and slew amount alternately to achieve an alignment with the turnout, as we did earlier in screen 36.

For the present we have left the mode 2 slew factor at its pre-set value of 100 (arrowed red). Notice that we can get a matched alignment over a somewhat longer length than before (arrowed blue). This is because for mode 2 the slewing maths approximate more closely to circular curves than for mode 1.

screen 45:

Then shorten the overall length to the join as before (screen 45).

screen 46:

Screen 46 shows our new mode 2 slewed return curve. Compared with the mode 1 version showing in screen 42, this one has a more sinuous alignment.

Generally for any given length a mode 2 slew will have easier curvature than mode 1, but care is needed to avoid a severe reverse curve which may cause buffer-locking problems. Bear in mind that we are here pushing the slew functions somewhat beyond their intended use, and each case of a return curve needs to be judged on its merits.

screen 47:

Before experimenting with the mode 2 slew factor, we can put this one on the background for comparison. I used the right-click menu in screen 47.

screen 48:

Now I lengthened the template again, and in screen 48 I selected the adjust > mouse actions: current > adjust mode 2 slew factor mouse action. (Sorry there's no shortcut for this one.)

screen 49:

In screen 49 you can see the effect of increasing the slew factor. At a factor of around 150 the curve becomes even more sinuous, and will require a reduced slew amount to restore the alignment (arrowed blue).

I have removed all timbering and track centre-lines on this screen to make things clear (control > background list menu item and untick the boxes for the background templates, showing here in blue).

screen 50:

In screen 50 you can see the effect of reducing the slew factor. At a factor of around 50 the curve becomes less sinuous, approximating more closely to a mode 1 slew. It will need an increased amount of slew to restore the alignment (arrowed blue).

Sensible values for the slew factor are in the range 5-200.

screen 51:

Finally a few screens of examples. In screen 51 I have inserted an improvised diamond-crossing into the slewed return curve, using the methods outlined in the diamond-crossing tutorial.

Just to make it more interesting, I arranged it to bridge the start point of the slew. Note that a similarly slewed track is needed for the branch road across the centre, and that its slew start position needs to be approximately co-incident with that in the main road (yellow arrows).

screen 52:

Then in screen 52 I added a single-slip road. This composite improvised template comprises seven overlaid templates, of which five cross into the return curve and so need to be slewed. Notice that in the case of the left-hand slip switch, this required the template to be swapped end-for-end, and its slew start position then has to be adjusted until the slew end marker (arrowed yellow) coincides with the other start markers.

screen 53:

Screen 53 shows the same thing without the clutter. The crossing angles are 1:6.5 .

screen 54:

Then in the final screen 54 I added a few more tracks for fun, and put an additional turnout in the return curve. Not a very likely-looking plan on reflection, but interesting to do!

That completes this tutorial. Thanks for reading this far.

Martin.

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