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                 Transition curves on parallel tracks.
     
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1st message | this message only posted: 13 May 2019 11:57
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from:
Godfrey Earnshaw
Crawley, United Kingdom

 

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I believe I understand the reason for transition curves, to ease a train into a curve, usually for passenger comfort:Q1) Should a transition curve occur on the exit of a curve also, in which case my second question is unnecessary.
If the answer to Q1 is no;
Q2) if there are two parallel tracks , one in each direction, does the opposite direction have it's transition at the other end and how do you make the tracks the correct distance apart?


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2nd message | this message only posted: 13 May 2019 13:40
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from:
Martin Wynne
West Of The Severn, United Kingdom



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Please do not send requests for help direct to me via email or PM.

Post your questions on the forum where everyone can see them and add helpful replies.
Godfrey Earnshaw wrote: I believe I understand the reason for transition curves, to ease a train into a curve, usually for passenger comfort:

Q1) Should a transition curve occur on the exit of a curve also
Hi Godfrey,

The main reason for using a transition curve on the prototype is to provide a length of cant gradient along which the rails can rise and fall.

Curved track needs to be canted (super-elevated) for safe high-speed running. That means the outer rail is higher than the inner rail. Straight track needs to have the rails level.

Clearly you can't join straight level track to canted curved track with a sudden jump in the rail level. A transition curve is introduced between them along which the radius is gradually reduced and the rails can gradually rise or fall. Typically in going from straight to a curve, the inner rail falls into the curve and the outer rail rises, so that the track centre-line remains level (or follows the ruling track gradient if there is one).

This applies just as much at the return to straight track as at the start of the curve. It also adds to passenger comfort, and again that applies just as much at the end of the curve as at the start.

If you model canted curves, all this applies equally to the model.

However the usual reason for using transition curves on a model is to disguise and mitigate the effect of unprototypically sharp model curves, even where all track is laid flat.

A join between a sharp fixed curve and dead straight track looks very severe and unprototypical (industrial sidings excepted.) It also causes a position of maximum sideways deflection between the vehicle couplings, gangway connections, buffer locking, etc., where one vehicle is on dead straight track and the next vehicle is on a sharp curve. Again this applies just as much at the exit from the curve.

Introducing even a short bit of transition curve between them can significantly reduce these effects.

Q2) if there are two parallel tracks, ... ... how do you make the tracks the correct distance apart?Templot will do this for you, if you select the auto-adjust option when using the tools > make double-track function:



To save me typing it all out again, here are the more information notes for that:



Reading through that, I think the notes for manual adjustments need updating (what doesn't?  :( ).

In most cases it would be better to create plain double track dummy templates at each end, and then use the make transition function between them.

For a crossover on a transition curve, the auto-adjust option is not available. It is better not to use the usual make crossover function. Instead make plain double track, and then insert a turnout in it. roam the turnout to the position where the turnout roads align. There will be a slight discontinuity at the join (which is the reason auto-adjust doesn't work), but well within normal construction tolerances.

cheers,

Martin.

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3rd message | this message only posted: 13 May 2019 14:04
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from:
Godfrey Earnshaw
Crawley, United Kingdom

 

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Thanks Martin
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